Depending on the next response, he could start using a bit of theocratic warfare and taking the tone of asking for guidance instead of directly challenging the truth. Using some questions that can't be answered , he might can get her to think more, but it's going to be tough, I would imagine the very appearance of his name in her inbox triggers all kinds of programming. The trick is to get under the programming to the mother who wants to help her son. Attacking the org is impersonal and triggers the deeply intrenched "debate" responses of circular reasoning. He will likely need to appeal to the mom in her, almost like he just fell off a bike (back in the day) and needs help fixing a wound.
You can't go head to head with the truth, maybe try asking unanswerable questions, lead her down the path.
Just like eating an elephant, you can't do it all at once...one little bite at a time.
WLG