So, because other people aren't respectful of other people's relationships means that he shouldn't? That's one of the most childish things I've read in a while. It goes back to that "if Bobby jumped off a bridge would you do it to?"
She's in a relationship. He's told her how he feels. It's now up to her. She either needs to leave her bf and date him or she needs to turn him away as a friend. Not only do they admit to liking each other, but they've already crossed that boundary of friendship with a kiss on the lips. If I was her boyfriend, that slut would be out on the curb so she could suck the next dick on her list.
Also, Sam, you need to think about one thing. Some guys are VERY vindictive. If a guy overstepped his boundary with my girl and KNEW that she and I were in a relationship, I wouldn't be very respectful anymore. You need to make sure this relationship isn't serious before you do something stupid. The more serious it is, the more likely it is that this guy might hurt you. No matter how tough you are, an aluminum bat is tougher.