You can try to dress it up as nicely as you want, it was unprofessional. You were unlikely to be the only one he talked to in that capacity that night and why didn't he pay others bills? It is irrelevant you became friends after. I am very good friends with a female colleague, we hang out socially all the time, I pay for her every time we're out socially because she is very dear to me but when we're working on a project or when we go to a conference she would never accept it nor would I offer because when you're a professional in a professional environment, you draw professional boundaries. You don't want to be the girl getting dinners from every guy she chats up.It is about how you want to be seen, it is all perception.