If I was running this for my group, I would turn that 1 in 6 chance of complete stone to just 1 hand or foot turns to stone: A stone hand is a penalty in combat and makes two-handed manipulations impossible. A stone leg makes movement and dex checks/saves difficult.
Future 1-in-6 failures would spread from the foot/hand up into the arm/leg and incur a greater penalty. That way the character can continue playing with the sense of urgency you are trying to cultivate. But maybe I'm just too soft-hearted, lol.