Nyc's advice and points are really good ones. I would also tend to think that there is probably an underlying issue here. His behavior towards her is abusive (I'm into D/s but his actions are NOT what healthy BDSM looks like) and her behavior towards you is incredibly hurtful and disrespectful. This could affect your health and safety, both emotionally and physically, and you're being given no choice and no time to adjust.
Whether you do as some have suggested and try to beat him at his own game (I'm not so sure about this idea, since if you're fumbling about as a beginner, which is normal, you might just make him look more competent and desirable... ugh) or insist she go with you to therapy ASAP or straight up move out until she figures this out... something drastic needs to happen sooner rather than later.
The major players. Me, under-30 bi female. Gia, girlfriend of 3+ years. Clay, new boyfriend/dom. Davis, ex/"it's complicated."
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy, Clay's partner.