Replying to Kinda worried

yo you guys gotta help me out, my dad is like 45 and trys to do rails and ski park but he has bad knees and im just worried he cant handle it. do i help him and try and make him better so he doesnt fall or just suggest he stops. like ive seen him smash his ribs on a rail on more then one occasion. He is a beast skier when it comes to the rest of the mountain and he does jumps and everything but it just seems like as i get better he wants to progress to and i dont want him to get hurt. So how should i help him.