Hard Drive Formatting

This should really be a simple question but it has been puzzling me for several days. I had a friend who sold a PC with the statement trhat the drive has been formatted. The buyer is upset because there is no operating system. The buyer stated that formatting a drive does not erase the data. I’ve racked my brain and talked to some other techs about this and we cannot figure out how to formatt the drive without deleting the data. Are we missing something? (my friend took the machine back to avoid any problems)