Thank you so much, everyone, for your feedback. My family and friends have given me the same advice (get the fuck away).
It's weird. I used to have no trouble dropping people like bad habits. I've broken off total contact with people I was involved with or just friends with for shit that's benign by comparison to all this. I haven't been in denial about any of the horrible things I've mentioned here... but I've had a hard time walking away. It's not getting easier.
The church ministers met with the kid and her parents, and my ex-boyfriend has been asked to step down from the youth group. He does not go to the discussion group his wife heads anymore. He doesn't go to the services. His wife is moving out of their house. He tried to kill himself last Tuesday.
He has come to the conclusion that he is sick and has many problems relating to love/sex addiction and procrastination. His wife just thinks they're fundamentally incompatible in their relationship and life goals. As far as I can tell, she doesn't recognize the cycle of abuse and the overwhelming disrespect that characterized their interactions. He does, though. It is a shame it took all of this for him to see it.
I took GalaGirl's suggestion and sent a note of support to his wife. She seemed touched.
The ministers wrote him a note of support. He's got support from his mom, the youth adviser friend, and a childhood friend of his, but he's leaning on me heavily. Even after all the shitty treatment, it still tears me up to see him suffering. Even though he is reaping what he sowed. Their life, which I thought was so beautiful when I met them, has crumbled. It is awful. Just awful.
@NovemberRain - I do not have a therapist at this time and cannot afford one, but I do have good friends who can provide support. There's also the ministers. I trust them more now that I see how they ultimately handled all of this. I'm thinking it would be good to meet with them.
I think I'd feel guilty to walk away, though I can't concretely say why. I'm in a lot deeper than I thought.