My heart goes out to you. It can be difficult to know what to do for a loved one who is failing. I had the same situation with my Mom (she passed a couple of months ago).
My Mom had no dietary restrictions but over several years she gradually lost interest in food. I did cook for her but in the end she ate very little. As for your FIL, between the heart failure and the diabetes there certainly would be dietary restrictions so, like others, I think you should check with his doctor.
It is always a difficult balancing act between the quality and quantity of life. In this I think it is best to take your cues from your FIL.
With my Mom, it made me
feel better to be able to *do* something. Ultimately, I remembered this line from Milton's poem On His Blindness
They also serve who only stand and wait
Simply being there is a gift. Your undemanding company is the food of love.