Is this a healthy solution to severe knee pain?

I know a man who is 75, 5’10” tall and very thin. Most of his active life he was a size 34 in the waist. He has severe knee pain and walks bent over then makes noises and pained facial expressions when he sits down. I’d say his waist is 30”. He says he “wants to lose another fifteen pounds” to help the pressure on his knees. Just how skinny is unhealthy, if at all, and why? I think he weighs 135 lb.