Or, at any rate, how to get someone back to riding their bike.
As good a starting point as any is to use their own ego. Do this by reference to their physical appearance. There are two approaches, and they are not distinguished by the ex-rider’s actual appearance but by personality type. For alpha sprinter-types, try, “Mate, you look awful.” For more modest climber-types try, “Well, you still look incredibly fit.” Either way, if you get it right, their reaction will be something along the lines of, “I’ll show them.”
If that’s not enough, dredge up the name of someone they used to be faster than. Explain how well this rider is going these days. “I say Lance, old chap, they say Cadel Evans is going to win the Tour…”…