We had several gallons of paint that were partially used and gave them to a friend. He mixed all the paints together and painted bedroom wall. The next day you could just peel the paint off in large pieces. What could have caused this and can it be prevented?

What type of wall was it? How as the wall prep for painting?
And what type of paint was it?

First of all were all the paints the same, latex / oil? They must be the same base. Also, were the cans inside or outside? If they were outside in your shed over winter and had been frozen they are no good. Did he prep the walls ie. Patch, sand and prime? Walls could have been dusty and needed wiping from the prep work. Also if the walls had a smooth finish and they were not sanded first that would cause peeling to happen.

"What could have caused this and can it be prevented" first thing is to be sure all the paint was the same type and none was ever frozen. Since the paint and likely the cans are gone we may never know the true answer but I'd guess there was some once frozen paint or oil mixed with latex.