people with late stage cancers often have enough trouble eating as is (a large fraction actually die of starvation), and getting them to eat anything is an accomplishment. So at that level, for a lot of post-metastasis patients, this will be happening naturally anyways.

Starvation does not equal ketosis. If cancer patients are suffering from nausea and lack of motivation to eat anything, they and their carers may not select high fat low carbohydrate foods that would promote and sustain ketosis and may instead choose simple and easy to digest carbohydrates and sugary treats.

(Your comment upvoted.)