Good friends who won't diet

I would leave them to it and keep them as friends.

They are choosing what they want to do.
They know what to do, but are not going to change.

Not easy and painful too?

What do you do with friends who are fanatic about dieting- obsessed even, and bordering on dangerous- wanting to save every penny, cutting small portions in half- and then going on massive walks without any food, snack or water? Both extremes are so dangerous. And both extremes totally refuse to see sense. My dad was like that, and a skeleton on his demise.

Not much you can do really, because no matter what you say they are not going to change their ways now. Riverwalk is right about their last few years.

I've heard it described as 'digging their own grave with their knives and forks'.

I think you can only broach the subject once and then it's up to them.

They won't die happy though - they could live for at least another decade getting progressively sicker and infirm.

I think it's gluttony in the main.

My sister is the same. She knows what she needs to do to improve her life but does nothing except complain about her poor quality of life .I have in the past suggested slimming clubs, walking, less drinking but now I don't bother. Any suggestions are taken as nagging so it is not worth it.
Bad lifestyle habits are hard to change, so much easier to go to the doctor and hope for a magic pill.
Enjoy your friends and accept it is their life.

Perhaps give them a low fat cookbook?are they happy people? Perhaps they just want to enjoy life's main pleasure - good food? Really difficult.

Omg! That's awful. Not sure if you can do anything though. Perhaps invite them to a meal where you cook something low fat but delicious? I would have said, let them die happy, but can you be happy with diabetes and painful knees.