The following are just some tips in how to speak to your Calvinist friend - mostly dealing with presuppositions I've often encountered in discussions with people.
Tip #1: Don't bring up passages of scripture that involve human action.
It seems like the false presupposition among many who interact with Calvinist theology (even if this is unintentional) is that Calvinism excludes any form of human action. Hence many, believing they refuting Calvinism, will run to passages in the Bible that ask people to confess their faith. "Ah, see," they'll say, "scripture says we have to do something, and since it asks us to do something, that must mean effectual grace is not involved."
Part of this may be the presupposition among some synergists that monergists (Calvinist, Lutheran, what have you) essentially believe that man is a robot, and all God has to do is hit a button and ZAP! man believes and is a good Christian. This, however, isn't the case. Some might be shocked to learn that monergists, be they Calvinist or otherwise, actually believe that man has a will. The question, however, is what we believe regarding the status of that will. That is, is it completely and absolutely free, or (as monergists and orthodox synergists believe) is it enslaved to sin? I could hold up a brick and say, "You are free to go up or down!", but so long as that brick is enslaved to the force of gravity and has nothing to free it from its grip, it will continue to go down the minute I let go. Hence we believe (as scripture teaches) that God's grace is required to free a man's will from the bondage of sin. That grace will then enable the man's will to be turned towards God, just as the Lord Christ said, "No one can come [literally has the power to come in the original Greek] to me unless the Father who sent me draws him..." (John 6:44).
So if a person throws a verse dealing with human action at a Calvinist, the Calvinist can only say, "Yes, I know, I agree with that." The question that must then be addressed, however, is where the ability for that action comes from. If you wish to have a discussion with your Calvinist friend, I might suggest coming from this angle.
Tip #2: Don't bring up popular passages to use against Calvinism.
This is a follow up of the previous post, but for a more specific problem. A lot of people, when they first encounter a Calvinist, want to throw out all the usual passages at them: John 3:16; 1 Timothy 2:4; 2 Peter 3:9; etc. If you have a Calvinist friend, or become acquainted with a Calvinist, I might suggest not throwing those verses at them. Not because they don't have an answer, mind you...rather, it's because they've probably dealt with all those verses at least a hundred times already. At the very least, you should not throw these verses at your Calvinist friend with the attitude of, "Aha! I've got you! This disproves your theology! Instakill!"
At the very least, what you could do is ask your Calvinist friend, "How do you explain these verses?" or something of that nature - by doing this, you'll come across as curious about their beliefs, and explaining the verses will be easier for them. If you want to go a step further for your friend, do some research on how Calvinists have been responding to these verses for literally centuries. You will then at least be able to adjust your arguments and not come across as someone who did their homework rather than just repeat what they heard a pastor say somewhere.
Tip #3: Don't ask them how they can do evangelism.
Another false presupposition is that, because God has predestined those who will be saved, evangelism is worthless and it's nonsensical for a Calvinist to evangelize. As I've discussed on this blog before, this notion that Calvinists can't consistently evangelize or do missionary work is contradicted by two things:
1) The precedent of history. Some of the greatest evangelists in history (John Bunyan, Jonathan Edwards, George Whitefield, Charles Spurgeon and countless others) were also Calvinists, and their theological beliefs offered no conflict with their missionary work or evangelism.
2) The simple facts behind our knowledge of God's election. That is, many people seem to forget that Calvinists don't know the identity of the elect. Many might be familiar with the John Carpenter film They Live, starring professional wrestler "Rowdy Roddy" Piper. In it, Piper plays a man who discovers sunglasses that permit him to identify aliens maintaining control over the earth. In a similar fashion, many people seem to think that Calvinists have special sunglasses which, putting them on, help them to see who are the elect and who are not. As we said before, this isn't the case. Calvinists don't have special glasses that they put on and then go into a crowded room to say, "I have come here to save God's elect and chew bubble gum...and I'm all out of bubble gum."
This might surprise some people, but all we do is follow God's command: we go out and preach the gospel to all people, knowing that God, by His grace, will save His sheep. In this manner, a Calvinist can evangelize or do missionary work and not feel his theology threatened in any way, shape or form.
Tip #4: Don't assume God's grace is forced on anyone.
Some people seem to be able to recognize both total depravity and irresistible grace, and yet seem unable to understand how the one relates to the other. That is, they seem to perceive that when a person is irresistibly called, that "irresistibleness" is reliant more upon force than it is upon enabling. This is where you get people saying that Calvinism believes people are "dragged into the kingdom," or that it is "divine rape," or even where people give that goofy bridge metaphor which says Calvinism believes there's a bridge to heaven and people are dragged kicking and screaming over that bridge.
This forgets that (as stated in the previous tip) part of irresistible grace is the idea that man's will is turned towards God. The regenerated heart will no longer have the God-hating inner nature of those still unregenerate. They will not be dragged kicking and screaming into heaven, but will earnestly desire heaven. That initial resistance to God will no longer be a dilemma, and to believe that it will is to misunderstand the morphing power of the Spirit upon the human heart.
In short, Calvinists do not believe God "forces" salvation upon anyone; God's grace serves to regenerate a person to have a heart of flesh rather than a heart of stone.
Tip #5: Don't assume the doctrines of grace end at the effectual calling.
Many people seem to assume that the doctrines of grace only involve the election and/or calling of the individual. I've had people ask me, if I'm elected, why don't I go out and murder somebody, since I'll be off the hook.
Of course, this fits more with the concept of "easy believism" than it does Reformed theology. The Doctrines of Grace do not teach that we are saved by prayers or a one-time human event, and on that one basis alone someone will be justified before God. Part of the doctrine of Perseverance of the Saints is the sanctification of the individual believer towards glorification, which will, of course, be perfect after the resurrection.
Tip #6: Don't call God maleficent, evil, wicked, mean, cruel, etc.
You may not like Calvinist soteriology, and you might have valid objections to bring up...however, you will get nowhere by name calling and declaring that Calvinists worship a different God entirely. I recognize there are many misrepresentations of Calvinism out there, however if you were to speak honestly with your Calvinist friend (or any knowledgeable Calvinist in general) to explain their concept of God, and you listen with an open and calm mind, you will find that their concept of God is no different than yours - if not more or just as Biblical. Your Calvinist friend will also demonstrate how he does not believe in a maleficent, evil, wicked, mean or cruel God.
Tip #7: Don't keep name-dropping Calvinist personalities.
Yes, we know about John Calvin. Yes, we know about John Piper. Yes, we know about James White, John MacArthur, and a lot of other big names in Calvinism, past and present. Maybe you don't agree with everything they say - that's fine, you'll discover a lot of Calvinists don't agree with everything they say.
However, don't throw them at your Calvinist friend as if you're refuting or defaming Calvinism in toto. Don't treat your friend as the whipping boy for any contentions you may have for another individual.