I find people react better if you explain why mIRC isn't suitable from a technical standpoint. By bringing the legal aspect it makes it sound like you're passing judgement, so in general the legal/moral argument becomes just that - an argument. Morality is an opinion and legality is dependent upon so many things, whereas the technical side is fact. That way you don't have to prove or presume anything about what they're downloading. That's not to say you can't say that there's a lot of copyrighted material being shared online, just that it's usually simpler if you don't use that as the reason for not helping someone.
