I don't think that's cheating, but it's definitely a red flag. A big one. It seems that he's seriously considering the end of your marriage. Of course that conversation being with his ex-wife can make you feel even worse, but I think the problem is not her, is him thinking about leaving. You should try to talk to him about that to figure out if it's worth to keep trying.

It’s certainly a betrayal. I understand needing to talk to someone but talking to your ex about something like that is taking it too far. Very poor judgment. If you can’t trust him not to do that I would wonder what else has been said.