OH is in a difficult place. She is dealing with a husband that has a disease as invasive as cancer.
I have a friend whose wife has Huntington's Disease. It is terrible and has now begun to impact her ability to think clearly. Would he be justified in walking away from this woman during her illness. I think not.
OH's husband has NOT been given a fair shake in my opinion. OH, we discussed this a lot on FB and I see no reason to rehash everything here. I think it is time to sit down with him and TELL HIM that you will no longer live with a person not taking responsibility for their own mental health. Let him know, as we previously discussed, that he has two weeks (or whatever time-frame you want) to have an appointment with a mental health professional. Play hard ball with him now.
The problem as I see it is OH has checked out of the marriage and is using this as a springboard(and a bit of an excuse) to walk away. If you want out, do it honestly and above board. Let him know you are leaving and close the door for good. If you still want a healthy recovered husband, give him a chance to make it right while still maintaining CLEARLY STATED AND FIRM BOUNDARIES.
I KNOW you are in a tough place. You have made it tougher on yourself. Today you can choose to make it easier. Your choice. If you are going to walk, do it for the right reasons and in a dignified and respectful manner. You have that right and nobody will fault you. You have the ABILITY to attack this problem in a far better way. The choice is yours OH.