Professor dating a graduate student
Better do so now than to have this professor sabotage your research.And, if you fixate on his weird relationship with the other student, it's not going to help you-it will only hurt you in the long run.And yes, he passed, got his degree, and got out of there.
Not that I've seen this scenario more than once or anything.)And even if there's no sexual relationship, the prof's chair needs to talk to him about appearances.
It's not appropriate for professors to do that regardless of their sexual identity.
Your not going to get anything by trying to uncover their affair at this point.
What matters is that he is in a position of power over the graduate student and it is completely inappropriate for him to have a sexual relationship with her because it's too easy to abuse his power.
But the fact is that you don't know if that is true - so you have to tread carefully on this one.