If he's just looking not starring then it's harmless. However if he's staring all the time and giving her lingering looks its inappropriate. If your out and you see a guy/girl you think is attractive, your gonna look. Its human nature. But when you do it to the same person all the time. It's wrong
There's a guy at work that did this to me for a long time. He would stare, use lots of eye contact, etc.. When I realized he was married. I started to ignore him and he finally stopped.
He may not have meant anything by it, but to me it was very misleading and not something that a person in a relationship should be doing.
If you are asking about it, does that mean you are slightly interested? Cos if I wasn't interested in the guy I wouldn't be asking if its OK. Sure there's nothing wrong with looking, more wrong would be acting on the intent I think if either of you are in a relationship.