1995 Saturn Ion My daughter didn't check the oil!

Question, happens a lot I know. The engine was knocking and she didn't check the oil. Got the oil changed and the mechanic said there was still oil but not much. She drove the mechanic to and from the shop, the engine still runs well, but there is the knock. (Very loud). I told her not to drive the car as it is probably ruined. Someone tolld me that it may not have enough lubricant to the pistons and recommended a procedure to help. Somthing about draining the oil out and putting something in, then draining that out and putting oil back in. Can you recommend something to be done to get the oil around the to see if this car is still driveable. We had just gotten it back from my oldest daughter who has driven it for many years. Youngest daughter is a new driver who ignored my orders to take it in for a oil change ASAP.