Tell your friend to tell his friend instead of water just fill his DEF tank with diesel so that he can increase his mileage
The above statement is total BS...just like the story your friends' friend told him....told you
Actually above statement if followed will end up damaging the DEF pump/nozzle/SCR so don't tell your friend to tell his friend.
Just tell your friend that his friend is full of BS and his story is BS because anything other than correct % of Urea (DEF) will cause friends' friend's truck to go into limp mode