Can you get a therapist to sign a statement that Rat's behavior wrt D's diet is harmful to D, and that it is recommended that XH can only see D (and S for that matter) without Rat around. Then can you take that back to your L? Lotta effort, I know, but maybe worth it.
Alternatively, how about the kill 'em with kindness approach. ADvise D that Rat's obsession may have more to do with her own weight gain, and let's just be really nice to Rat because she's clearly insecure.
So those are my two, really different cents worth!
