Please, please do not suggest treatments for your friend, especially those suggested by people on the internet.

People on the internet may mean well and show you interesting things, but your friend's medical care is best managed by his oncologist/primary care physician. Interfering with their work will likely do more harm than good.

Definitely encourage your friend to ask any and all questions about his care; communication between doctors and patients is helpful for everyone involved.

Self-medication, whether by diet or drugs, is a terrible idea. People die doing this.