I don't think pursuing "friendship" with a former romantic partner is ever a good idea, even with someone who is mentally healthy. It leads to someone becoming unhappy.

I agree with you that this man is emotionally fragile and in a ridiculously stressful situation. However, nothing you do will change that. His bond with his parents is what it is. He should not be expressing such morbid thoughts to you, while refusing your help. This guy has kind of a "martyr complex." It's best you don't provide an audience for that.

I don't know how things work in every country. Still, there is probably a nursing home that could care for his parents. He could visit daily and help with their care, but he'ld get breaks. His parents raised him to be as he is. He probably won't consider altering the arrangement he's in. I doubt you really want to take on caring for these two old people, even part of the time. The whole set up is unhealthy. You really have to just let go.

If he contacts you at some point specifically asking for a favor or to speak with you, then you could respond in a kind, compassionate way. But, starting now, leave the ball in his court. If he sends you despairing messages of how he wants to die, tell him, "I'm so sorry you're having such a tough time. Let me know if I can help in any way." Then leave it to him to initiate anything further.

Taking care of his parents is not going to kill him. Many people care for multiple needy family members. I understand he's lonely. I suspect he would still be a lonely man, if both his parents were gone to heaven and he were free. He is chronically depressed. That probably will never change.

Knowing him so long, you're sad to see his life being pretty tragic. Don't revolve your thoughts around his sad life to fill up some emptiness in your own life. Find other friends you can be involved with on a healthier basis. Don't decide you have to make yourself sad because he is sad. There is no way you are going to become his savior. He won't let you. He has decided against that. Respect his right to decide for himself.

If helping needy people is something you love to do, find some worthwhile charity you can volunteer with. They will put your energy and abilities to good use. You might even end up saving someone's life. Some situations you have to leave to God or to fate, when a person won't open the door to help. If you are so inclined, you keep the person in your prayers, and you move on. Don't stay stuck ruminating about something that's not for you to control. Let go.