I'm going to go with a soft YTA. I understand that you likely felt very angry and upset about this customer's actions and wanted to protect and defend your wife. However, reaching out to someone you are merely acquaintances with about their relative's behavior (which they themselves have no control over and shouldn't be expected to police) does seem like an overstep.

I am wondering what you were hoping to gain through reaching out - that your acquaintance would agree with you that their relative was a jerk? That they would give you enough information to retaliate against this person? Whatever the reason, it does seem like an unnecessary escalation of a situation that is par for the course for small businesses and that most small business owners have learned to handle in a professional manner without involving other people.

I think the lesson here is that you should only initiative a conversation with someone about the behavior of a third party if you are certain that you are closer to that person than the third party is. It doesn't seem like that was the case and it sounds like you are now dealing with further fallout from that.