Grapefruit juice is most likely not compatible with some of the medications he is on. It has nothing to do with liver flush. His doctor has most likely never heard of liver flush.
But, it doesn't have to be grapefruit juice for the liver flush!
You can flush the liver with just about any other fruit juice ... from Lemon, Lime, Pomegranate, Kiwi, Sour apples, Sour Oranges, Clementines, Mandarins, Ploms, sour grapes, etc. This list is endless!
The main problem is not grapefruit juice, as that can be easily replaced.
The main problem is Liver Flush on itself ... as it burdens kidneys, with or without grapefruit juice.
I would suggest slow, gradual cleanse ... like taking one or several tbsp of olive oil + as much lemon juice the first thing every morning.
If no negative reaction, he can take some of it every morning the rest of his life.