I can really help you with this one! My friends have done this a lot & its fun everytime. Get some extra slick skateboard wax, it wont come off easy. Its made to eliminate friction when skaters do grinds, also works good as rosin for musical instruments. Just slick up the bump, you cant get arrested for it, but he can get arrested for disturbing the peace.
I've been in trouble for this many times but there is nothing illegal about making something slick (haha). I've been in jail overnight a few times for it but all i learned is that you can get good sleep there. If the person gets hurt theres no way you can be found out plus its not a serious fall, at best this person will get minor scrapes & bruises. Also the slick will stay there so he will most likely find a new spot.
parrafin wax, petroleum jelly (vaseline), olive oil. just needs to be slick & itll harden up so u can use it easy. also makes good rosin, hairgel, crayons, pastel paint, etc. more olive oil for slickness, vaseline will make it easier to use, parrafin wax is what holds it together. you can put it in an old deoterant sick container, thats what skaters do.