This. It is very dependent on the type of cancer. There is a lot of research on this. For a little context, I was a pediatric neurology professor for a while, and have been on a low carb diet for a decade. Much of the department did low carb, as did much of the oncology dept. Many kids with epilepsy are put on keto as well with great effect. I did a deep dive into low carb research before starting and keeping with the diet, and found a lot about using it for cancer therapy. I'm not sure what has changed in the last 10 years, but the above abstract looks promising. With a GBM, they probably don't have much to lose. *This is not medical advice, I'm not an MD (I was a BME doing epilepsy research), have them check with their Dr.

The keto diet was actually developed specifically to help people with epilepsy[0].

[0]: https://epilepsysociety.org.uk/about-epilepsy/treatment/keto...