The one statement I didn't see in the OP is - 'they' are looking for a change in their plans. If 'they' aren't doing this for themselves, they will never follow through.
Anyway....
You haven't mentioned their financial status but if they are 'comfortable' there are several 'prepared meal' companies that do Keto meals and prepackage and send them to the front door of your friends. This way they could have their variety in meals and control their intake if they would just agree to follow the plans and not cheat on the side with high carb or processed foods. If their knees are bad, they would probably appreciate not having dishes and pans to deal with - just eat out of the individual containers and throw away. If they agree to comply for 30 days, the benefits they see will convince them to continue on for longer. If they are serious after that, either they could enlist the DIL to prepare similar meals or else learn the rules themselves.
The one statement I didn't see in the OP is - 'they' are looking for a change in their plans. If 'they' aren't doing this for themselves, they will never follow through.
Anyway....
You haven't mentioned their financial status but if they are 'comfortable' there are several 'prepared meal' companies that do Keto meals and prepackage and send them to the front door of your friends. This way they could have their variety in meals and control their intake if they would just agree to follow the plans and not cheat on the side with high carb or processed foods. If their knees are bad, they would probably appreciate not having dishes and pans to deal with - just eat out of the individual containers and throw away. If they agree to comply for 30 days, the benefits they see will convince them to continue on for longer. If they are serious after that, either they could enlist the DIL to prepare similar meals or else learn the rules themselves.