Girls always try this sort of thing (although not all).
They like a Mr Reliable someone who offers them security and consistency. This is who you are to her. While she wants all the perks of a steady sensible boyfriend she isn't going to give them back. She knows you like so she knows she can keep you to heel until she has had enough of exiciting and slightly naughtier boys than yourself. You are her banker basically. I would recommend trying to get both of them into bed or failing that see your new lady if your friend doesnt like it is she really a friend?