If in this context means it's not something that is certain. When in this context means it's going to happen, but hasn't happened yet. So they kind of mean different things in those two sentences, and it depends on what you're trying to say. Hope that helped :)

Zak is correct that "if" means it might happen, and "when" means it is going to happen. Because of this, and the subject of the sentence, I would add that you should never say "when your friend breaks up with his girlfriend" unless that friend has already told you they are going to break up with their girlfriend. It would be considered VERY rude. Saying that while your friend still has that girlfriend would be like telling them you know their relationship is going to fail.