It is unfortunately normal for a person who is prone to such severe depression to try to take the easy way out like ending a relationship or going to a new job. They look for outside reasons for their unhappiness when really is an internal struggle. Being his friends is of course ridiculous. This is his way of healing which is of course superficial. He needs to work on himself and he doesn't realize that I guess. You can't make one decision and not be depressed. The choice to sort out the marriage isn't going to happen if he has convinced himself that the marriage is the problem. There is nothing you can do about this unless he is willing to consider that this isn't the problem. You can ask to meet with him and a professional but he still has to sort this out and what his mental health is like. I would have a talk with him and tell him that you don't want to be his friend.
If this has been helpful press accept