Have you considered that her relationship with her sig. other may not be all she wishes it were. Is she living with the guy? If not, you could decide to be a contender for her affections. Try and find out the status of that relationship. If it's not totally solid . . . move in.

Here's what I wouldn't do: Don't go spilling your guts in the interest of "being honest." There are plenty of times in life when "being honest" is totally inappropriate. This may well be one of those times.

Try to find out if this is a happy young woman, content in her current relationship with future plans involving her significant other. Maybe she's not.