The risks of untreated pyelonephritis (kidney infection) is scarring to the kidneys and in infrequent cases it can become systemic and result in septicaemia, which can indeed be fatal. Encourage him to take his antibiotics.
Three possibilities:
1. Your friend doesn't really have a kidney infection and will be fine.
2. Your friend's immune system will wipe out the infection, even without the antibiotics.
3. The infection will spread (sepsis), and your friend will end up in intensive care with a good chance of dying. Whether your friend lives, your friend may need dialysis every other day (about two hours tethered to a machine) until a kidney transplant is available, which will likely take several years.
the infection will become bigger and he'll have serious health problems.
try convincing your friend, it's not nice to have a big infection on your internal organs.