At Triablogue, Paul has posted an item on free will and God's advance knowledge including an answer to the popular non-Calvinist argument: "Just because God knows in advance that X will happen doesn't mean God causes or controls that X to happen." (my paraphrase)
As Paul points out, that argument misses the point. While I like aspects of what Paul wrote, let me put my own spin on this, namely - how can you respond to your friend who uses this argument with you?
First, you can provide some disclaimers. These disclaimers can help remove any straw men that may exist between the two of you. Those disclaimer can be, for example:
1) God's knowledge of future event X is not itself the cause of the future event X. If God's knowledge of the future caused the future to be, then God's knowledge of the future would necessarily entail the future existing as known. We don't allege this. We don't claim God's knowledge causes the future to be.
2) Simply person A knowing the future doesn't entail person A causing the future to be. We know this, because sometimes God tells men what the future will be. At that point, the men know the future, but - of course - this doesn't mean that the men cause the future event of which they have advance knowledge.
Once these disclaimers have been provided, you can go on to explain the force of the argument.
1) God's infallible knowledge of future event X implies that future event X will happen with absolute certainty. God can't be wrong. Thus, God's infallible knowledge of future event X means that future event X is guaranteed to happen.
2) If an event is absolutely certain to happen, it cannot be otherwise. This may seem trivial, but it is an important point. If God knows that X will happen, it is certain to happen, and thus cannot be otherwise.
3) An event's absolute certainty implies an inability of actors to do otherwise. If an event cannot be otherwise, a person cannot bring about the event being otherwise; for if a person could bring about the event being otherwise, then the event could be otherwise. But the event cannot be otherwise, because the event is absolutely certain to be as foreseen.
4) An event's absolute certainty implies an absence of "Libertarian" Free Will with respect to the event. If we take as an example a particular choice of a free agent, such as man, and if we say this particular choice is known in advance to God and consequently is absolutely certain to happen, then - as we have shown above - the person making this choice will not be able to choose otherwise. But this absence of ability to choose otherwise contradicts the "Libertarian" account of free will. In other words, such a choice is not "free" according to the "Libertarian" model.
Some Immediate Conclusion
1) Because God's knowledge of the future is absolutely complete, we know that there is no such thing as Libertarian Free Will. There may be free will of some kind, but not of the Libertarian kind, because people are not able to do otherwise than has been foreseen.
2) But, per our earlier disclaimer, God's knowledge is not itself the cause of the absence of Libertarian Free Will. In other words, what ensures that people cannot do otherwise is not simply God's knowledge of what will happen. After all, we can have advance knowledge, but no one would reasonably say that our advance knowledge is the cause.
1) Whatever kind of free will we have, it cannot be "Libertarian" free will. There's no reason that the term "free will" has to be thrown out, just because we can demonstrate that we lack an ability to do otherwise. There's still a very real sense in which some human acts are "free will" acts, and others are either involuntary or coerced. This would be a definition of "free will" that is compatible with extensive Divine sovereignty, not one that is opposed to it.
2) There is a larger explanation for both God's knowledge and our actions. Since God's knowledge itself does not explain why we choose X and not Y, we should look to a larger explanation. The larger explanation is one that explains both God's knowledge and our actions. The correct explanation to this is God's Providence, his most holy and wise and powerful preserving and governing all His creatures and all their actions.