How long can a urinary tract infection be left untreated before it will become dangerous? this is for a friend who won't see the doctor, and i worry.

Depends: Infection in the urinary bladder alone may not become dangerous even if left untreated. There is always a risk that infection could spread to the kidneys. If kidneys are infected, then it can become dangerous in a matter of hours or days, if there is obstruction to urine flow and the patient is diabetic. It is best to get treatment at the earliest.