I don’t see a clear malpractice claim from that exchange. Producing non-responsive material is not grounds for clawback, and his attorney may simply have determined there wasn’t any privileged information in the texts.
Not to mention that the fact the overeager attorney on the other side called it a mistake doesn’t mean it was one. It may have actually been a strategy intended to bury the actual good stuff, in which case it’s arguably sanctionable but not malpractice.