I think that the issue you are dealing with is called "The openness of God theology," which has become popular in the last several years.

A friend send me this link today, and I’ll pass it on to you.

http://www.probe.org/docs/openview.html

We had an incident in our church a couple of years ago. A highly respected teacher stood and taught that prayer changes God’s mind. He did not choose his words wisely, and he seemed to adhere to the "openness of God" position discussed in the article above.

I would suggest that you look at my lesson on the immutability of God.

In summary I would say that God has made it clear (in texts like Jeremiah 18) that He will relent concerning promised judgment when true repentance results from the prophecy of impending judgment. Both Moses (Exodus 32) and Jonah (Jonah 4) understood that God is unchanging. Jonah knew that God would relent, because God doesn’t change (His merciful and compassionate nature). He keeps His covenant promises. Jonah was angry with God because He acted consistently (by relenting when Nineveh repented). Moses pled with God, based upon His covenant promises and His unchanging nature.