In these situations, you have to make a choice between wielding power and having influence. Power would mean pulling out some financial stops (i.e., making her college payments conditional on her staying home with you) , while influence would mean making her want to spend time with you. With pseudo-adults (okay, she’s a legal adult, but you know what I mean) exerting power seldom yields a good result, so I’d opt for influence.
Include her and her boyfriend, even if you think he’s a skanky dirt weasel, every single time you invite her over. Hold your nose, but do it. She thinks of them as a package deal, so you should, too. If you want to see her alone, ask her out for a mother/daughter shopping excursion (or whatever she likes to do). The goal is to spend as much time as you are able with her so that you can be the voice of reason, and so that she knows you have her back when the whole thing falls apart. The odds are good that it will, so just hang in there.