I don't know what I'd do in your shoes, Greenkit.
I just wanted to say that MissMoo may find it far-fetched wrt to men having fuck-pads, or shag-pads, or whatever, in addition to the familial home.
They do.
It may be out of your experience, and it may not be what's going on here, but it is not necessarily untrue.
I'm also not sure about this "none of your business" stance, either. I think that may often be given as an answer by the prurient-but-cowardly. There is a fine balance between giving people (not just young people) their autonomy and their privacy, and between shrugging your shoulders and allowing harm to continue in front of your eyes/in full knowledge.
It is interesting that there is a poster responding who has a positive experience of such a relationship. It is a reminder (to me, anyway) that mostly, allowing young people to negotiate relationships for themselves works out fine: at worst, they lose a little time, they get a little hurt, they gain a lot of wisdom.
Of course, sometimes it doesn't work out like that. For some reason, there are other forces and motivations at work. Perhaps the only way to distinguish between the two situations is to have more information? And maybe that suggests what you might do? That is, keep talking and listening to your daughter and what she say?
I don't really have any advice, Greenkit. But I wanted to offer some kind of validation/listening. I've noticed that there can be a bit of a tendency on mn to absolutely trash a woman's perceptions and views. I'm coming more and more to think of that as aform of violence, actually. And quite pernicious.