I had the same experience. My first few flushes were nothing but water, but with green grit. I did not get stones out till my fourth flush. So perhaps your friend will see the results if he keeps on flushing. I believe that he may also have stones that are broken in small pieces and he may not have seen them.
Please let him know that he may want to try again, that is the only way to clean your body.
I am curious, why did he decide to do the flush? Normally people who experience some health issues they come to this forum and then realize that this will help them.
Tell him that he is not the only person who did not get any stones the first time.