You're not a cockblocker. Although it is not your responsibility to maintain somebody else's relationship, as a friend you should let your friend know before hand. Tell him, "Listen, the girl that you're kickin it with just told me she was feeling me and gave me her number." If he still fukks with her after that, and you do end up smashing, then he has to take the L. Some dudes are emotional and would chose a hoe over a bro, so if he does that then your friendship wasn't worth shyt anyway. However, I wouldn't smash a friend's girlfriend, but if it's just some chick that he's trying to bag but didn't close the deal, all is fair in the game. But to not say anything and just smash is considered to be underhanded. It sounds like you've done this a couple of times. You don't want that type of reputation. Makes your friends not trust you around their women.