I have not couch surfed myself, but have very close friends who frequently host couch surfers from Europe ( in the US) and used to travel that way, as well. To my knowledge, it never got dangerous for them, either traveling or hosting, and often it was very pleasant (they are a family with a teenage daughter, so they don't want dangerous either). They engaged in extensive e-mailing with potential travelers, to make sure they have a good match. They had some lovely visitors from Europe whom I met, several times just two young women traveling together, so I don't have a bad feeling about a young couple using that service. That said, I think communication with the potential host is essential, and if there is any kind of "weird vibe" going on, it's best to opt out.
Good luck to your daughter and her BF in their travels!
PS. I think Ajax's advice was right on.