Do try to get therapy, it's been a great help for me at least. I never would be in the fix I am in now if I had still been seeing my therapist. Having someone who is on the "outside" is very good when you internalize as much as we do. I'm much too close to my own problems to see them clearly. Having some one professional is great to bounce your own feelings off of, and help you understand what is going on with yourself.
I've done the "friends with benefits" thing in the past too. The problem with it for me was that I'm pretty much the faithful type and so I didn't really go looking for something better. It was comfortable for a long time and that's where your friend may be. However, his lady friend may not be willing to give up the relationship, even if he is. Mine got vey jealous, even though it was never a question of anything besides friendship between us. If your friend doesn't like conflict, he may stay in the relationship because it is easier then facing that, he must like her or he wouldn't be there at all.