From what I remember, your bf's mother sounds like she shares some of MrsHD's traits. What say you try to get some respect out of her? What do you think your chances are of that? When she gets into one of her moods, all bets are off, right? When she's like that, what do you think you can do? Don't tell me you will just walk away and leave her alone because to make this exercise relevant to HD's sitch, you can't do that. You are stuck in the "relationship" and have to find a way to make it work.
What do you recommend that will not be "catering to her, coddling her, and playing by her rules?" How do you propose rattling her cage that will make an impact sufficient to change her behavior and bring the two of you closer? Just telling her that you will not tolerate her poor behavior is not sufficient because she will call your bluff. Then what will you do to back it up? Remember, you can't just walk away and abandon the relationship. Filing for D is not the answer either because HD has not decided to go down that road yet. What do you suggest?