It must be hard to have to share business with xh and see him like that. I think you did the responsible thing. Has he been seeing the kids?
His using prescription drugs is undoubtedly due to his depression and his inability to deal with his guilt. My H has to drink and take something every night to sleep, causing him to often not show up for work (his own company). Luckily he has good people working for him and he's somehow able to stay on top of things. But I wonder how long the drinking and sleep aids will work for him before he begins abusing prescription drugs as well. Knowing my H, the time he spends alone with his thoughts are too painful for him to deal with, so he needs to numb the pain.
Hope your x hasn't done any irreversible damage to the company. Maybe him being forced to leave will turn out to be a good thing - one of the many events that will lead to him eventually hitting bottom. But doesn't sound like that will happen any time soon. Keep living your life like you've been doing. Hang in there.