I can tell right now this is going to be a tense place for me to be operating in. I think that your girl banging your friends mom is awesome and to be honest with you I would be trying to work my way in.
I know you have feelings for her and I would be sitting with the overarching question, is my relationship with her truly any different now that I know my girl likes to eat box? I think where it could get hairy (no pun intended) is if you get involved with them in bed, if jealousy creeps in, if someone in the relationship gets the idea that other parties are welcome in bed, etc.