Are there any messages popping up telling him it won't work? If so, post it on here and maybe I can see what's going on nurga. Your question is vague, so I'm sure I need a lot more than "it won't run."
If he got Parallels illegally, make sure he doesn't register his version. If he does, the program won't work.
Also, make sure that he hasn't moved the files that it needs to boot up the Windows OS. If he has moved them, tell him to put it back into the original place where it was found.
Those are just a couple of troubleshooting tips. Just get back at me with a more solid explination and I will help you crack diz sheet mang.