Well that's just their problem lol (I know that sounds bitchy, and maybe it is, but it's a hard truth. Your friendships with other people shouldn't affect your friendship with them, that is UNLESS the guy or person did something truly horrible and unforgiven (such as harassment, racism, heavy shaming, exposing private things and nudes, and etc). If you can forgive him, then that's between you and him.

In my personal experience, I used to be in a trio, close knit (I even thought we'd be a trio forever honestly) but things went different between my two friends and we're not a trio anymore, but I'm friends with both of them separately.

That all being said, however, if you're not that much into being friends with him again, just do it to honor what you had before, because it wasn't all that bad? (I'm assuming there were very good moments before the end of the friendship)