I would question how "stuck" he is in living with HER, in particular, due to his finances and her providing transportation. He could share housing and transportation costs with someone else (with whom he's not in a boyfriend/girlfriend relationship) to help cut cost. Also, do you have any concerns about him leaning on you for financial and transportation support???
I think you may be closer to the truth in thinking you're in a type of "other woman" situation. The statement: "I'm really not happy with her and am only staying because.........." is often an "other woman" scenario.
If nothing else, he sounds like someone who is quite ingrained in the "victim" role, (the 'Poor me. I'm helpless to do anything about my life on my own. Rescue me! Rescue me from that awful other person!!") which is not a healthy one under any circumstance.