Most cancers are dependent on glucose as a fuel source, as they typically use 200× more than most other cells in the body.

If you can, convince your friend to go on a zero-carb, zero-sugar diet that even cuts out artificial sweeteners (as they can also screw with metabolism). Get them to try to hammer their A1C down to 3.0 or less for 3+ months, so they go into extended periods of ketosis. A continuous glucose monitor can help identify spikes, which can also be moderated with 2tbsp of vinegar before each meal.