Who_Am_I wrote:
Everyone who is saying "don't be a p****, just nail her" and "he's just a boyfriend, it's not like they're married": I hope that if you ever find someone, they cheat on you, and then turn around and say "well it's not like we were married or anything".
I'm not suggesting that he actively chases her, just if he does have feelings after getting to know said girl, tell her. Dating is to find a person to marry, I'd say it'd only be unethical if she was engaged. I'm also not suggesting for them to date behind the boyfriend's back, either. If she likes him more, she leaves the boyfriend, if she doesn't leave, nothing should happen between them. She's given the choice of one or the other. She can do what she wishes.
I know lots of people, too, make basically....contingency plans, and keep people in mind as an alternative in case trouble comes in a relationship. My sister had an abusive boyfriend, for whatever reason, and was talking to a "nice guy" on the side. Once things erupted with abusive a**hole boyfriend, she went out with "nice guy." People'll date someone they don't like, or liked initially but don't like now for whatever reason, and then they'll start looking around for new people. Not that that's wise per se, but it's their way of correcting their problem they created by not being honest with themselves and what their actual wishes are from a relationship. Or they're not able to be honest with their partner they don't like (sometimes not their fault, for example, abuse) and end things, and stick around just because they hate being alone.
Anyway, if I were you, I'd pretend she's not staring at you and don't acknowledge her existence. There. That's what I'd actually do in real life because I'm a p****.