I feel that way for several reasons...some of which were discussed off board and will leave it to OH to bring up.
But for starters...depression is a serious disease and it robs a person of exactly what OH is looking for from her husband...motivation and direction. IF he had any other disease, we would be more understanding. Depression is still very poorly understood and still carries with it a stigma that people should just get over it. It is as real as cancer. He is UNABLE to do certain things right now...not just unwilling. This is the part of the vows that have to do with "in sickness and in health."
He doesn't know she wants to leave. He KNOWS she likes to complain and then do nothing. He also knows, by her actions that she is not in love with him. Imagine how that might make a depressed person feel.
OH has not figured out in her own mind that she wants to be married to him. Inasmuch, this entire issue is being used as a tool to justify walking away.
He needs help. He doesn't need to be abandoned.
I feel for OH. I understand her pain. I also KNOW that she hasn't sat down with him...perhaps with the help of another family member and let him know that he has X amount of time to SEEK help. He also doesn't know that she will be there at the end of this...heck, she doesn't even know. There is already a foot out the door.
Yes, he needs to work on his health. She needs to be supportive of that or go on her way. She gets to set reasonable boundaries and deadlines that should be reviewed with a mental health professional BEFORE presenting them to him.
Cat, imagine that a woman was here complaining that her husband wanted to walk away from her because she couldn't work due to ovarian cancer. What would we say to her??? I KNOW I would be furious for her. OH's husband has a disease that is every bit as debilitating and just as difficult to manage. At the very least, he deserves her best effort and a clearly laid out plan of action.
Just my opinion.

Don't go shaking the [Bleep!] tree and expect an angel to fall out.

Liars lie and cheaters cheat...know it and don't be surprised. Protect yourself.