Perhaps, she realizes at stage 4 that her odds of beating it may not be that good and she'd rather have quality than quantity for the remainder of her time here?

I respect that, and that will be my choice also. Most people who undergo chemotherapy are so sick they feel like doing absolutely nothing and end up dying anyway.

Therefore, instead of looking at it as self-sabotage, maybe look at it as an informed decision about how she's choosing to go out, like this lady did:

https://www.facebook.com/DrivingMissNorma/

As far as self-sabotage goes, it is a learned behavior and therefore can be unlearned ... but like anything else involving change, it's a difficult process - especially if it was learned early on and accompanied by feelings of powerlessness and hopelessness, such as a child growing up in a horridly abusive environment.

As painful as it may be for you, I think the best thing you can do is respect her wishes and offer your support the best you can even though you may not understand or agree.

Sincerely,
Pfrog!