I think what others have said here more than capture my own feelings on the topic. The co-facilitator was insensitive and likely broke your confidentiality, but probably not intentionally and certainly didn't appear to mean to cause you any harm. Therapists are human, make mistakes, and should own up to them when confronted with them.

Honestly, if you run a group, therapists have a much harder time with this issue than one-on-one therapists, because they may be privy to additional information about each person that the group as a whole doesn't know. That's why groups can be harder for therapists to conduct, but it's no excuse for not being vigilant for respecting one's clients' privacy.

I'm glad you talked to your therapist about this issue... I think that was the right course of action to take.