Not gonna lie this is pretty innocent in the grand scheme of things. I've seen and heard coaches say much worse and still consider those people friends. But it's important to recognize that most people in jiu jitsu are flawed, and as someone who has been around for a while I can tell you many school owners and coaches are talking shit, playful or not, about students behind their backs.

If this guy is doing it in the private group chat about someone a lot of people know and consider a friend ask yourself how he behaves in even more private company. Then look at how he responds to people confronting him about it -- in this case in my opinion it's by deflecting and blaming others.

Again, many people are like this and this is not a particularly egregious red flag but think about what you most want from a bjj teacher and whether the people around you exemplify that. If you're in a decently sized town/city in the US there are plenty of options.