Genocide was kind of on to something. Crush the laxative tablets, but sprinkle the crushed product onto the gum. If you were smart and got some type of white tablet, the gum would just appear to have a little extra sugar on it. I seriously doubt your friend would just chew the gum if it weren't in a wrapper familiar to him.
I don't really condone this, and the others are right about it being childish, but I suppose I can help.