Something more than the obvious?

This sort of thing happened to two of my roommates, my first reaction was that Ruth knew that you liked him she shouldn't have done anything, but you gave her the go ahead. I think you need to see if she is serious about this guy, and tell her your feelings before you tell him. That might put a strain on your relationship, but not as much as if you started dating him behind her back. If you tell this guy that you like him and he doesn't like you back you might lose two friends over it. So I would ask myself which is more important.