Billy likes Sara, and he does all these things to prove his feelings. Sara starts saying how amazing of a friend billy is, and how she never wants to change that. He's just been friend-zoned. He wanted to date her and be in a relationship with her, but she just wanted to be friends. Lol lame story I know :3
If you liked your good friend as much more than a friend, you would ask them if they want to take the friendship to the next level. If they say they just want to stay friends; dont want to ruin the friendship etc, that means that you have been "Friend zoned".