Ah, that girl, I know that girl (plenty of 'em floating around). I'd have to say she most likely cheated on you. Talking to you about how sexy "S" was a huge hint that she wanted to break off your relationship and get with "S". No good girlfriend goes on and on about how sexy your male friends are to your face without some (not so subtle) agenda. It's also possible she wanted you to get pissed off and show her how "manly" you are by kicking "S"'s ass. It always amazes me how women tell us to be more "civilized" yet at the back of some of their minds they still want us to smack 'em over the head and drag them into the cave by their hair, so to speak. But I digress.
On the subject of good girlfriends/relationships, nobody without an (again, not so subtle) agenda has to ask their partner how they feel about being cheated on. Nobody likes to be cheated on, and no normal person should feel like they have to bring it up with their partner to see how they feel about it.
So yeah, probably cheated on you, and while it's easy to put most of the blame on your ex, some of the blame definitely should rest on your friend for encouraging their "relationship" in the first place. Bottom line: making a pass at your friend's girl is always out of bounds.