Husband newly sleeved - MIL driving me nuts!!!

I think your MIL sounds absolutely dreadful and her constant nagging would drive me crazy too.  I would take the route of scaring her a bit.  I would tell her that your husband needs his rest and his protein in order to have his heart (which is a muscle, and muscles need protein) work well after the surgery.  This will hopefully quiet her down.  Tell her that he is following the doctors orders and if he stops following those and starts to follow her advice and something terrible happens, it would be just awful. That might make her back away very quickly. 

Elina, we're thinking alike! That is pretty much what I ended up telling her. I said that if he does not get enough protein his body will turn to his organs to get it. I'm not even positive that's what really happens - lol - but it sounded good and it shut her up! ;-)