does this sound legit? i wanna try it

A friend of mine played in a casino, held pocket 6s. Flop was 9 6 6. He hit his quads, while his opponent had pocket 9s. They went all in, and J 9 came off on the turn and river. By my math, that's quad 9s and quad 6s in the same hand. Apparently this is impossible though, can you explain why not?