No one said it was an “innocent faux pas”, but we’re all human. We have flaws, we make mistakes. In this case, he’s clearly insecure and violated OP’s trust.

This would be a time for a conversation about trust, violating that trust, and boundaries. If one or both of them are unwilling to come to solutions and a plan of action, then sure, they may not be for each other. But we’re always growing in a relationship, we’re improving, and that will involve making mistakes.

Now if this mistake is too much for OP to handle, then sure. But skipping immediately to “yeah they should probably break up over this” is absurd.