answer:The theory is that having tires with different amounts of tread wear (the new one has no wear, the old one is somewhat worn down) is inherently unsafe. It has to do with the way unmatched tires will handle—unmatched tires my pull and veer in unpredictable directions. NOthing will happen unless she’s driving and the car doesn’t handle as well, or pulls to one side, or steers strangely. Lots of people do what your friend did, with no consequences. But there are risks. How good a driver is she?