People have fallen in love with others while in relationships with people. Unfortunately (or fortunately, depending on the point of view), being in a relationship means very little a lot of times. I'm not saying he should actively pursue anything though, all I said is that if he does do that, as long as it's consensual, of course, she was the one in the relationship and the one who wanted to cheat, or wanted out, wtv. It's not really the users place to worry about it. That's just how things go, most of the times. Of course, it's also your prerogative to chose whether you care or not, try to pursue it or not. Would I try? I barely try anything on single people, but if someone in a relationship invites me to go out and something more than friendship seems to exist, I don't think I would care.
I say this as someone who's been cheated on, take it as you will.