What should I do ?!!!!!!?

Ok this guy asked my best friend for a picture of her in a bikini. She called him a perv and said no. This guy really embarrassed her and now we want revenge. The boy is an older brother of a boy on my little brother's baseball team and they have the world series at the beach. Well we can't be there and it is the prefect opportunity to get back at him. Our friend is going thou and we asked him to do something to him and he said he would but how do we take it from there if we wont be there to know if he did it or not?