K.U.
I would encourage her to get some professional counseling for herself - she could consider marriage counseling if her husband is willing to go along but if he isn't, she could at least go alone, and maybe she will figure out some solutions to the problems she is having and get some insight into how she can turn things around for herself, one way or the other. It's sad that she thinks she would be better off dying from cancer - my mother just passed away this past summer from complications from leukemia (which did not respond to treatment, and was incredibly difficult to have to watch) and there isn't anything I wouldn't do to have her back. Remind her that her life does have meaning, and she needs to remain strong because her son needs her, and there are many many people who do care about her and want her to get the help she needs.
Not that I think medication solves everything, but along with a professional therapist/counselor, maybe being on antidepressant meds would help. She should also talk to her doctor about why she is feeling the way she is and if there could be something medical (hormones, etc.) to blame.