If you value either one's friendship at all, you need to back off.
If they were together for 6months, there are some underlying feelings there, whether its admitted or not.
If he is interested in you, he will approach you - at that time, you can feel out your girl friend's response.
I would let him date a couple of girls before going out with him to let feelings die down. There are too many other fish in the sea and a relationship with him, at best, would make things awkward.