The old gent's doctor should have given him some dietary guidelines and should also have had him consult with a dietitian. Depending on his weight and pre-existing conditions, such as high blood pressure, there may be lots of little tweaks to make. Since his kidneys have stopped working, lots of stuff can build up in his blood between dialysis sessions, and some of the build-up can be toxic or fatal.
If you Google on "dialysis diet" you will get LOTS of hits, including collections of recipes, but your GF should be aware that she'll need the guidance of a dietitian to tailor those recipes to the needs of the patient.