I understand your situation. Many years ago I had a FWB who was without question the most highly-sexed woman I have ever encountered. She was in constant need, but was also extraordinarily generous as a sexual partner.
When she went through the menopause as a result of a total hysterectomy, her sex drive disappeared completely. She changed from spending most of her waking hours thinking of sex into being sexless.
By that stage we were friends, but without benefits, as I had begun a stable, long-term, romantic relationship. Our contact slowly petered out (not at my wish), and I do not know how things panned out for her.
My current partner's experience is the opposite and she has become more sexual since undergoing a natural menopause. She regards it as a real bonus to not spend one week in four menstruating, and feels much more liberated as a result.
I don't know where this leaves the OP. I see sex as the currency of love in a relationship, and my partner and I certainly would not want to be without it. Even when age and its physical limits prevail, I cannot see us doing without a certain level of bodily intimacy. I see sex primarily as a giving action; we have sex to give pleasure to each other. If that were not so, we could just as easily masturbate to satisfy our own desires.
If I ever lose my desire to have sex, I cannot see myself ever losing the desire to give pleasure to my partner, and I'd happily bring her to orgasm with fingers, toys, tongue, etc., simply because it is a wonderful act to bring joy to another person. That way of thinking affects my view of the OP's issue: of course we should not engage in sex acts that we don't want, but why wouldn't you want to be a source of physical pleasure to your loved one? In order to maintain the relationship is it not better to have some level of sexual intimacy, instead of living like brother and sister?
It's a tough problem: her partner did not sign up for a sexless relationship, but she ought not to feel forced into acts she no longer desires. For me, it would result in a very frank discussion about sexual needs, open relationships, FWBs, etc., as I have a very primal need for some sort of sexual relationship.
I hope she can find balance that works both for her, and for her partner.