She used up all the oxygen in the pocket of air trapped in the car with her. There were 4 nearby houses The Swimmer could have stopped at before going to his hotel for a shower, shave and nap.
That is murder. But he is a Kennedy. So instead of a long prison sentence, which he deserved, he ended up getting (a few years ago) a big hug from former President GHW Bush, who was as stupid as those rocks on which the smashed car is resting.