Could be any number of things, either way unless he has a good friend with some extra testing hardware laying around it will cost him.
Easiest and probably most expensive thing (if its not under warrenty) is to send it to HP and let them fix it.
If you have another motherboard, CPU, and RAM you can use to swap in with the ones he has now you can elminate them one by one. If his CPU, RAM, and MOBO are all good... something like his 120mm case fan not working may cause the system not to allow boot. Something could be loose.... It could be alot of things.