God knew that Peter would choose to deny him three times.
God has prophesied the future. If God doesn't know than, he is presumption or he forces to future to occur. But God has prophesied man's sin repeatedly, including: Israel's sin, Peter's sin, Sin of man in the last days, sin of man following the 1000 year reign of Christ.
If God prophesied sin, and yet didn't cause it Himself, then God must know the future. If God did cause it Himself, then how can it be sin?
I believe that when God says He is the Alpha and Omega, that He is already the Omega. He exists at all points in time simultaneously. That is why comments are made in scripture like Jesus' "Before Moses I AM" instead of "Befoe Moses I was". That is why future events are often spoken of in past tense in scripture.
The above was the last sentence of my post. It is important that you see it again.
Now, what would the open theist say about God prophesying sin. It's an excellent question. This will be off the top of my head.
Perhaps he'd say that God works through people, and when they let God down then he's aware of it in the present because of settled events in the future that He knows will come certainly come to pass.
For example, if I know that Iran starts a nuclear holocaust in the next decade, then I'm able to say that Iran does develop a nuclear weapon.
The question is this: what did the Lord know about the future that was certain and settled that would have required Peter to have denied him early that morning?
I can't think of a single thing.