Yikes!! Poor little bugger.... you don't say how old he is? But at this point I would absolutely begin to refuse care. If he can no longer participate in day-to-day activities (and I would say you are there), then he cannot be there. If you aren't quite ready to get tough with her yet, then for the child's sake I would throw as much fibre and liquid at him as possible. Let him drink a gallon of apple-prune, if you can get in into him.
Personally I am disgusted that this mom isn't taking it serious enough to get to the bleeding grocery store to pick up some prune juice. But it's ok for you to put pills up his little bum? Grrrrrrr.
And yes, girls; the poop talk goes on. I talked to two parents today about the subject!!