@marsiey Welcome to Mayo Clinic Connect. It's always concerning when we see test results changing without a clear cause. You write that your husband has been going through different medical procedures related to his lung cancer, plus cardiology concerns. Any and all of those may upset his system to produce reduced eGFR. Medications, not eating well/enough, dehydration all could work together to lower his kidney function, not pointing to one particular thing.

It's always a great idea to eat healthy, no matter our medical concerns. Most of us will tell you diet plays a critical role in helping s be the best we can be relating to kidney function! Watch the potassium, phosphorous, calcium, sugar contents of things. Eat good protein, minimize stress, get moderate exercise when possible. Stay away from processed meats like deli meat, etc. And if he already has a modified diet based on other health issues, remember to include them!
Ginger