I saw a horse today with a crack running half way up its hoof. It looked like it should be painful, does anyone know if it is or not? will it heal?

I believe it normally depends on how deep the crack is and what caused it. With corrective shoeing and treatment it should heal as the hoof is like a fingernail and will regrow in one piece so long as the issue is corrected that caused the crack. There are various ways to stop the crack from growing and shoeing methods to prevent it from returning and aid healing.