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.
It is about doing the right thing, sometimes we don't have to look at the consequences, like putting ourselves in the same shoe. That is a help to the girl and her future, if the father is going to kill his future then the innocent girl shouldn't be affected, if telling her will help the issue then go ahead. Clear up the air about her fathers addict and ways to combat addiction, talk to her about it and it is for she not to mention the part you told her to the father, both of you has to be reasonable to save yourselves and avoid any later consequences.