Ooh, juicy. I would probably "bump" into her and see if the dude was actually her brother/cousin/something platonic. If she seems flustered I'd be suspicious and I'd probably confront her about it at a later date, but honestly I try not to get involved in those sorts of things. I would probably want to tell my friend but wouldn't, and it would probably suck, a lot. Loyalty is one thing but that sort of thing is none of my business, really. I would give her the benefit of the doubt at first, though. Good question!