I do believe it is your job, because you will be doing something that could potentially impact her life in a HUGE way. You say that you would be upset if you were married and your husband did it to you..so why does that give you the right to put another woman in the same position when you know you wouldn't like it? Just because you may not know or ever meet his wife, doesn't give you the right to hurt her vicariously through her husband and his actions. That's kind of like saying Hitler isn't responsible for killing the jews because he talked someone else into doing it for him (not an exactly analogy, just making a point here--hope that is not offensive to anyone). Anything you do that involves another person, you should think about the greater impact of others around you, otherwise you are being incredibly selfish. And just because some of your friends say they would do the same thing (by flirting with the guy or whatever), doesn't make it the right thing to do.