P.S.
From someone who has been where you are, the best thing I can tell you is to follow your gut instincts. Yes, men and women can be friends, but if your gut tells you something is wrong then follow it.
Men don't always know (or admit) when a women is interested in them. Try inviting her over to your home for dinner and befriend her. Maybe she can call you for her crises. If she is not interested in your friendship then that tells you something.
Tell him that "YOU" have a problem with his relationship with her and if it continues _______________. If you make it your problem it makes it easier to for men to handle.
Remember what they say..."Keep your friends close and your ememies closer"
Good luck