Faithfulness is a relationship if your in a relationship your committed to that person meaning you shouldnt be doing stuff like that cause you sorta promised yourself to that person, now if your friends with benifit's that's different but even so that wouldnt mean your commited so really this depend's on the relationship but if you like someone and you believe your ment for them then there shouldnt be anyone else your making moves on.
Relationships can get very messy, and I wouldn't want to cause trouble for someone else unless absolutely necessary. But if a friend's boyfriend or girlfriend made a pass at me, I wouldn't think twice about telling them- because what sort of friend would I be if I didn't? Yes, you run the risk of making things awkward between the two of you and yes, they'll probably be angry at first- or even for a long time. Even so, once you've told them the truth it's up to them what they do about it; if they choose to stay with the person anyway and end up getting cheated on, they can't accuse you of not warning them. However, if you aren't friends with their partner I'd say leave it and just stay away from the person who made the pass at you.