Hmm let me think....
There is only 5 things it can be, monitor , CPU, mobo, RAM, PSU. :haha:
Can you be more specific, is the computer beeping now when you/he starts it up? How does he know it's ok if he can't see anything? A good clean POST will give you 1 long beep, if that's all he gets then maybe it is just the monitor. Can he borrow a working monitor to try it?