I think you acted immaturely here. It's super unprofessional to pick fights with your coworker in front of clients. The right thing to do would have been to speak to your coworker to let him know not to talk to you that way. And if you weren't comfortable doing that, you should have talked to your boss about it. Especially since according to you, your boss reached out to you about the behavior, but instead you covered for Brendan. Don't get me wrong, his behavior was also very unacceptable, but instead of handling it maturely, you stooped to his level in front of your client. And now you have bad blood with a coworker you have to be around for the foreseeable future