How does serious stress affect someone with ADHD?

If you know my story, my spouse and I have been separated for months. He has untreated ADHD. I am trying to balance co-parenting with him and maintain a friendly and supportive relationship. I also come here to continue to work out some stuff from our pretty traumatic relationship. 

Here is my question: What does serious emotional and environmental stress do to someone with ADHD? I believe he went through it a lot as an undiagnosed teenager (parents who were hyper-critical and mostly absent) and again as an adult. I saw that with me, keeping a steady job, the care and support of children, and the deterioration of our marriage increasingly and significantly affected his stress levels. And then our separation and now some financial insecurity is causing him a great deal of stress. I have seen him go from someone who is affected by untreated ADHD to someone who truly appears to be having trouble functioning, who can hardly move or make himself deal with anything, who is having trouble taking care of himself, and a social person who now never sees people. He cries frequently. He is losing interest in seeing our children for more than short periods (I think he does not want them to see him like this). I am concerned about him as a person and my children's father and have been urging him to see a psychologist or psychiatrist, but so far, no dice. I actually think he is on the precipice of a pretty severe breakdown in some way. 

I would appreciate any thoughts from folks with ADHD or their significant others in terms of how extreme stress may affect those with it. How can those with ADHD manage it? Many thanks.