well, it's not really mom's job to fix him any more than it is yours. he's an adult human not a defective auto part we return to the factory. so - let her deal with this situation with her son as she sees fit. as long as that does not require ANYTHING from you. like you said, you're tapped out.
you aren't leaving HIM at HIS worst, you are saving yourself before you get to YOUR worst. being the spouse of an addict is impairing, diminishing, depleting. you can't stop him.....you CAN save yourself more misery.