He's not really using Mary as a beard....if that were true, she wouldn't know she was being used. She knows and she's on board with it....Plus, like Billy said, she's one of the nicest girls in their class and it's a shame nobody asked her to prom...He's helping a girl who probably wanted to go to prom but wouldn't go unless she had a date because of self-consciousness and she's fully aware of the situation. It's an extremely nice gesture in my book...When I was in sixth grade, I was at a school dance and my friend (Angie) the girl with Down Syndrome was there and I asked her to dance a slow dance with me....it was the right thing to do because it made her happy. Yes, Billy may have altruistic reasons for asking Mary, but she's fully aware of the whole situation.