perhaps John needs to look for a new department or role at work. Some quiet backoffice where he can be productive without too much inter-personal contact.
From the description of his job that is what John already does.
I advocate for the 1 on 1 talk. Express that while he appreciates her trying to include him in the office banter, the way in which she is doing it actually makes him uncomfortable. No threats of escalation or "I'll take this to HR if you don't stop." Just a thanks, but no thanks, kind of conversation.