This is a classic girl thing. We have to try to see conflict differently. The conflict in this case is because he is genuinely vulnerable. He has made the mistake of putting his eggs all in one basket. This is not good for him and in this case it has not worked out well.
He may well threaten all sorts if he loses you. We can't get you through this problem if we minimise the situation he is in. It is a bad situation; he deserves help and support to improve it. You will not go so far as keeping him as your partner, or living with him, so those options are off the table. But as a friend, you can work with his mother to make things better for him.
Hs mother needs to be told, in absolutely non-negotiable terms, that the marriage has ended. But you are willing to act in friendship to help him move on, because of the friendship you two had before.
His mother actually needs some help. She probably does not want to deal with a deeply unhappy and troubled adult son. She thought it was all solved, as he had married you. That's fallen apart and she is sad and frustrated. But nice she knows that the family needs a Plan B, and if you insist kindly but clearly that this is where it is going, she may be prepared to do her share.
For you to survive, I think you have to be able to detach a little bit and not experience her frustration as pressure on you. It's true, it's a fact, that he is in a bad place and its about to get worse. What you have to believe is that it is soluble and that there is a happy life out there for him.