Camp right now is the best place for her so its probably best not to tell any of the higher ups because they’ll just show her the door and tell whatever school shes in/going to. (Being there means she isn’t with him and probably doesn’t have contact with him -I’m assuming cell phones aren’t allowed but today you never know). I also think the fact that she mentioned it not once but twice was her saying that she knows it might be wrong but isn’t sure and wants someones approval. If its possible try to become friendly with her maybe even out of camp (what you’re able to do). At least this way she can have a solid support (even if its just for the summer). Theres a good chance that if “they’re getting married,” she isn’t doing this for a few days. If she brings it up again, try to find out if anyone in her family knows about it.
But thats just my (not-very-experienced-and-not-very-smart) talking. The other part is that I really think you should contact Rabbi Wallerstein from Ohr Naava. He deals with this stuff every day (sadly) and he can probably tell you what to do. (The number is 718-OHR-NAAV(A) and the website is ohrnaava.net. It looks like theres a way to contact him through the site.)
Lots of Hatzlacha