Going to do a "Roast" of a fellow co worker...?!
Answers:
don't roast him roast his job, his car, his shoes what-have-you, but don't go into anything personal, the usual things will work as they will be expected
"i don't know what we'll do without you? .. who'll make the coffee now!"
"invaluable member of the team! .. we tried to sell you but no one offered .. valueless maybe!"
etc
That's a pretty risky idea. Why not just order in a bunch of pizzas and have people bring snacks and sodas and send him off in style with a cake. Get a card that everyone can sign and that is easy for him to take with him.