Well, I haven’t been in your situation, but I’ve been in one similar to your daughter’s. My advice?
First off, she’s 18. At this point, she really has the right to choose her own friends and associates. Also, it sounds as though she has her head screwed on straight.
I have been in a similar situation to the one she is in, though. In college, I didn’t really date, partly because I didn’t have the time to put into a serious relationship: I was in school full time in a rather intense program while working three jobs. Also, I was still figuring out who I was and what I wanted out of life. I did have a number of friends though, and one was a gay man. I had made it clear to him what I believed about homosexuality and the human person–i.e., what the Church believes, that to have SSA isn’t a sin, but acting on it is, human beings are much more than our sexual appetites, etc. I was one of the first people he came out to because he knew I believed that. Why? Well, bear in mind that we lived in the South, and that the few people he’d discussed it with were of various branches of Christianity that taught that because he had SSA, he was automatically going to Hell, which of course makes no logical sense at all.
Obviously, I was careful not to get my heart involved beyond caring for this guy as I would any other friend. We would hang out together, as your daughter does, in a mixed group of friends. Occasionally, if a school function implied that we “ought” to have a date, we’d go together. (This was especially great if the function–a concert, say–was in a questionable area of town and ended rather late.)
I’m married now, and live in another area entirely, but if I did live in the town in which this guy lives my husband certainly wouldn’t be threatened by our friendship, nor would he expect us to break it off. Of course, this guy and I would still be careful: he and I wouldn’t hang out one-on-one, just as I wouldn’t now with any man. However, if my husband felt threatened by meeting a group of four or five of my friends for dinner and drinks on occasion, and one of them happened to be a gay man…well, I’d say the problem was his, and not my friend’s. (For the record, DH wouldn’t have a problem with this at all.) My husband has female friends at work, and he does the same thing: he and they sometimes go out for lunch in a mixed group, and I have no problem with that, but he’d never go out to lunch one-on-one with a female friend, just as I wouldn’t meet this guy by myself for lunch or coffee.
One of the reasons this friendship has worked out as well as it has, and would work out well in the future if we were in the same area, would be that we were both quite honest from the beginning–he about his SSA, and me about the Church’s teaching. If, for example, he and a partner were to come into town and wanted to stay at our place, he wouldn’t be surprised or offended by our request that they stay in separate rooms because he already knows our beliefs about SSA behavior; we wouldn’t be springing them on him in the heat of the moment, so to speak. He respects our beliefs, and we respect, albeit disagree with, his.
All relationships, friendships included, must be founded on respect and courtesy. It would be one thing if this man were practicing that lifestyle, came over to your house, and insisted upon making out with his partner in front of you. (Frankly, that would be obnoxious behavior in anyone, never mind the sexes of the people involved!) However, by demonstrating that she respects him and cares about him as a human being…well, you never know what might happen. Perhaps it may spark his interest in the Church, even! In any case, I don’t understand the mentality that because a person has a certain attraction, one shouldn’t be their friend. It would be one thing if they were disrespectful of the would-be friend’s beliefs or rude about their Faith–we aren’t required to put up with rude or abusive behavior–but if they aren’t, why not be a friend? Surely that’s part of being Christ to everyone?