Did he ever actually stick up for you even once before that? Because if he didn't, this wasn't some sudden betrayal, it was just the first time it showed. Guys like that pick the side with more power and they do it early, you just don't notice until it costs you something. My advice is don't confront him or try to get an explanation, you won't get one that means anything and it just keeps you tied to the whole mess. Cut him loose, keep your head down around your mom for a while, and pay more attention to who actually shows up when things are bad. That's the only test that matters.

Man... I had a buddy down in Wichita who went through almost the same thing. He made some dumb joke on Twitter back in like 2012 when he was barely out of high school, and then eight years later some customer dug it up and his boss let him go the same week. The part that got me was he never even got a chance to say hey, I was 19 and being an idiot, here's what I actually meant. He just got walked out. I don't know if there should be a hard time limit on old posts, but it bugs me that there's no room for a person to just explain themselves anymore. People change a lot in ten years, and it feels like we all forgot that.