First you will need a lot of scaffolding blocks. Go on top friends house and build up until you only have no more scaffolding blocks(you will also need 8 other blocks of any type that does not fall). Once you only have 4 scaffolding blocks, put the normal blocks beside the top blocks(all sides and not the corners) and some blocks on top of the ones you just placed. Put lava in the litte hole and jump off(use a water bucket). Wait until the player comes.

He/She will probably break on of the scaffolding block and all of the scaffolding blocks will break and lava will come down. They will go crazy trying to protech there house and if they do not see it, well lava will come down no problem.