Why does my friend date this fat chick?

Okay so one of my friends/teamates is dating this chick whose like 260 pounds at least and im not exgagerrating to make her sound more than she is. I mean liteally. Hes the captain of our high school basketball team and all the prep chicks like him but he dates this fat loser? She was bragging to me about the great sex they have all the time. Gross. I asked him about it and he said yeah shes fun. Does he date her to use her maybe as a joke? I cant think of any possible reasons!