Yeah, he can leave it on whenever.
But first, he should understand the reasons why its good to leave it on or keep it off.
Every time you turn your PC off, the solders cool down. Whenever you turn it on, they get hot again.
Turning it on and off will eventually make the solders crack, making it impossible to operate further.
Keeping it on 24/7 helps these problems, but makes more on its own.
What his computer was doing, being left on all the time, makes his fans collect dust bunnies and may eventually cause overheating.
It also makes things like a hard drive wear away with friction after a while.
So really, its his choice, I keep mine on all the time, but I have three fans, his most likely has one.
I don't think keeping it on all the time made this problem, but it may cause others in the future and is, once again, really all up to him.
-Graboy