The timing belt was replaced, but the car still won't start.

He replaced the timing idler gear (think that's what it was called) because it had seized up. He also replaced the timing belt. He set the timing, but the car won't start. He has checked the fuel. We have fuel. He has checked compression. We have that. He has checked to see if there is a spark. There is. What else could it possibly be? He swears he did the timing correctly, so he refuses to check it again. I'm hoping you can point me in the right direction so I can either tell him a new place to look, or tell him to look at the timing again. Thank you very much!