My friend was in a head-on collision, had essentially a broken neck and upper vertebrae, and had to stay in the hospital for more than a week until he was well enough for orthopedic surgery.

" fractured thoracic vertebrae, multiple lacerations and contusions, as well as injuries to his left arm and lower leg,"

stabilized in order for the hospital to allow him to leave. He may have wanted to leave as soon as possible for any number of reasons. We just don't have that information.

Is what I'm guessing.

Often termed hairline fractures...Discharge would be expected. You can't compare serious fractures that are displaced or at risk of displacing (ie, those that are already showing some nerve impingement or at risk of doing so) as would have been the case for your friend. Apples and oranges...