Again, I think you've offered really good advice to your sis. As you say, if there was a quick fix, everyone would be doing it. Fasting is too aggressive for the toxic and degenerated state that most folks with CRC find themselves in. Enhanced nutrition not starvation is what is required to strengthen your body during detoxification.
The die off and the fasting stimulated cleansing would indeed put one in bed for some time and would severely stress the eliminative organs. This wouldn't necessarily just resolve itself with a happy ending. It is not wise to push the eliminative organs to eliminate unless you are very healthy.
Vegetable juices can be used as nutritional and detoxification support but not as a fast. That is also too aggressive in my opinion. Once you have become ill, the cleansing must be approached gradually and alongside a good gentle yeast purge as well.