I Know Her Secret

I never really had a close relationship with my dad. He left my mom when I was two, and we only see each other every other weekend. He's always been distant, and not very good at forming relationships. He's had no less than five different girlfriends since breaking up with my mom. All in their twenties, even though he is 44 by now. I think they have all been with him just because of the money, and I'm pretty sure he has been cheating on most of them. He is currently dating a girl who is 24 yo. But unlike before, he seems genuinely in love with this girl. Thing is, I know she is cheating on him. She had a one night stand with one of my friends. Now I'm genuinely conflicted about what to do. Should I ignore it and let my dad have a taste of his own medicine? Should I tell him? Or should I confront his cheating girlfriend about it?