I think that really simplifies the problem. The question you are asking is very deep and how time works is hard to imagine.
So your problem is that God knows the future, so what if God tells someone their future and then that that person wishes to make a different choice....what happens then?
I'll use this example for God and.... lets call him Bob. I think the problem is that you assume that God has a very simple knowledge of the future. By this I mean that your stories only allows God knowledge of the event in question and not the events that lead up to it. God doesn't just know that Bob eats Wheaties on Friday, but also whether this decision is based on Him (God) telling Bob that he (Bob) will eat them prior to the event. So what God knows will happen is the future that happens because He tells Bob.
So the next problem is that if God tells Bob he will eat X and so Bob chooses Y, it makes Gods statement wrong. If God therefore (knowing Bob will change his mind) says Bob will eat Y, this may make Bob change his mind to X (which he was going to eat anyway).
So one answer might be that some times God CAN'T always tell us what WILL happen, even though He knows. This doesn't mean God doesn't exist, it could just be how Reality works. So the answer is that maybe God can't tell Bob what he will eat on Friday.
To be honest it is a very hard question to properly imagine, let alone answer. For example God knows the future and tells (or doesn't tell) Bob all in one instant reality and any 'change' God or Bob takes are in fact never changes. Many this is a paradox just like the rock paradox and you just have to say that God doesn't do illogical things. God doesn't tell people things that they change their minds about (unless it is to influence their decision perhaps), just like God never tries to make a rock He can't move.