It could be many factors. What does he mean by "crash" - it freezes up and he needs to force close it? He disconnects?
If it's affecting MSN too then I doubt it's an mIRC issue. Perhaps he has a virus, or if it's that he is disconnecting from the Internet perhaps someone is attacking him. For the former, send him to this thread
as it has a number of good resources.
Is he doing anything specific before the crash (i.e. running any commands or receiving a file etc.)?
What version of mIRC is he using? What OS? What scripts, if any, is he running? It's hard to help someone through their friend, tell them to come here and register themselves