Hi Annise ~ I see no other way out of this other than therapy. He has GOT to get her to see that unless they get therapy, this relationship is going to end, and she's going to be left alone. Perhaps if she understands that she's going to be left completely alone, that will open her eyes.
I'm sure you son would rather just give up and leave -- i imagine he's had about enough of this, but I hope he's williing to give counseling a try. he sounds like a good guy, thank God.
Have him tell her that it's counseling or he's gone, and she'll be left high and dry and that YOU will NOT be bailing her out. Maybe that will do it.
I wish everyone the very best. God blsss and please keep us posted. Hugs, Lee

PS -- this is called "tough love."