@enchantedspirit, thank you for your comment! The former best friend is my son's father who is a pretty cool guy and easy going with anything. That's why everything was so confusing for me. Him as well does not understand why his best friend got so mad at him and refuses to see him. Even so, with them two not being friends anymore, I imagined we could solve this like grown ups and meet anywhere else but my building.
I suspected there was an underlying condition or something else causing this, but can't know it for a fact because he doesn't want us around. Depression, dementia, late stage cancer, all these passed through my mind.
And because he is important to me, I chose not to let go, but to give him time and space.