This is legit.
Johnny Carson’s piano playing guest canceled at the last minute due to an accident, so Johnny asked if anyone in the audience played piano. This guy, David Tolley, raised his hand, so Johnny had him come down an play.
Tolley reports via the YouTube page that this was legit, it wasn’t a set-up – he honestly volunteered from the audience and it wasn’t planned.
On December 19, 1985, David was just another guy in the “Tonight Show” studio audience when Johnny Carson said, “Horatio Gutierrez (a classical pianist) couldn’t come tonight because he accidentally shut his hand in a car door and didn’t want to chance playing the Tonight Show. Can anybody here play the piano?” David volunteered; he played “Memory” (from the Cats musical). The next day, David got tons of phone calls — everything from playing on cruise ships to acting in movies. Since then, David’s career has gone up and down.