Don't intervene, you might end up putting yourself in a situation where you might start regretting your actions...One of my principles Is that I never get involved in other peoples family issues, even though it might seem like you need to speak up and tell his daughter what is actually going on that is not going to be a good move..sooner or later she is going to find out, things like this don't stay hidden for long
I understand that op is worried but trying to relate this to this man daughter may lead to another issue, letting this girl to know this Migjt come with consequences, the op has a good intention but his good intentions might end up making him look like a bad person, in this kind of situation, advise the op to stay out of trouble, this might look like nothing to him but once he make that move in his mind, he will understand that this matter is actually a family matter, the best option for the op is for him to act like he never saw anything, if I were to be him I will try to talk to the man in question instead of relating this to the daughter.