Ive spent 17 years single parenting twin daughters...13 of those celibate. You can sacrifice everything for those you love if they matter that much to you. She needs to weigh the value of her relationship to her....and decide how important her kink really is...if its more valuable than her relationship she very much should move on and divorce.
She stated that her husband and her attempted an open relationship but he couldnt find comfort in that so your mention of Polygamy is pointless....an open relationship doesnt even imply the significance of polyamory....let alone plural marriage (which is what polygamy means despite redditors constant misuse of it in reference to polyamory)
And again....divorce is an option...if she values her needs and desires more than her relationship. Thats okay....but its important to note that a relationship CAN be more complex, deep, and significant...than just bedroom compatibility. Just as she wishes him to engage in her kinks despite his lack of desire....she can just as easily (not easy for either at all) move on from these impulses, urges, and predilections.
I get that this is /r/BDSMcommunity.... but the real dynamics of EVERY relationship is sacrifice at some level.