This situation is very complicated. His mother is probably limited in her ability to care for herself due to the illness; however, she may be more self-supporting than your husband thinks she is. My suggestion is that your husband and his mother begin therapy, see a case manager, or a doctor, if they are not doing this together already, and determine her level of functionality. Once an outside source determines her level functionality, they can set goals and a plan for his mother to become more self-supporting, based on her degree of functioning.
The illness has probably destroyed his mother’s life but it should not be allowed to destroy his life or your life. Your husband does, however, have a moral obligation to help her to the best of his ability but it would be wrong to do this at the expense of his life, your life and your marriage. There is a very good likelihood that his mother can do more than she is doing for herself now, and that may include living on her own, respecting your privacy, or even earning her own pocket money. Your husband is probably doing too much for her, most likely because of his guilt. But because of how he has treated her, she may have never been brought to her highest level of functionality.
In summary, suggest to your husband that he and his mother visit with a mental health professional that can assess her level of ability and functionality. Based on the assessment, he and his mother should strive to have her become more independent. I suspect she is much more capable of caring for herself than your husband has allowed her to be. I also suspect that the relief that would be gained from her doing more for herself, even if it was just a little, would be more than enough to make you happy. I hope this helps. Thanks for writing.