This is something I worry about myself (even though I've got a few years left before I'll deal with this)
There comes a point where the grown ups need to be grown ups and set things straight.
You say she doesn't like him, is that "like Like" him? or not even like him as a friend?
Has she told the guy to knock it off? And I mean really told him, politely, but leaving no question at all that it will never happen.
Sometimes I think our kids are a little too concerned about hurting someones feelings and it prevents them from telling the truth(they just want to be nice to everyone). It's hard, but there is a way to be firm and truthful, while still being polite.
Maybe just buy him a copy of the movie "She's just not that into you"?