I would first talk to his doctors or someone at the treatment center about this. YOu have to tell him, it should not make a difference in his recovery because he should be doing it for himself.
If you do not want to be in the middle then you say no I do not want to be in the middle. I can see why she wants supervised visits with the kids, he has used drugs-what mother would let her kids go see someone that used drugs. He is recovering but it is in a controlled environment.
If she wants to get on with her life you can not blame her, she seems to be looking out for herself, and in a wierd way she is looking out for him. Sell the house to pay off the debit. That is great.
No matter what your son needs to talk to a lawyer to make sure that everything is correct in what she is saying. But the courts will have him do supervised visits they could also ask him to do regular drug tests to make sure he is not using when he gets out of rehab.