Tell them its a rewarding hobby and it will keep them off the streets and doing drugs. (Depending on how old your friend is) It worked for my friends parents!
Tell them it will keep him occupied lol. Almost always occupied. Also it teaches responsiblity of taking care of another living being. Although 20 is a little small for salt. Maybe he can do a mini reef with Damsels or something. He should really shoot for a 40 gallon. It's best to start out 30 gallons and bigger. Just don't mention $$$ or anything negative.