For Christmas, he sneaks her out of the hospital to take her downtown to see all the trees lit up and buy her a present.
It's not clear if she dies because of this, or merely shortly after.
Otonashi decides to dedicate his life to helping others. He returns to school.
He turns his grades around and... is that a college student ID he has in his hands?
He'll never get to use it, as the train he's on has an accident and he dies.