Where can I go for some specific dietary information?

When someone is going from a very restricted diet to a “normal” one, I know it can be important to transition gradually while the body adjusts.

What if—

(a) the present diet is jail food, notoriously cheap, fatty, and ill-prepared, and not especially nourishing, and

(b) the person who’s being transitioned is undergoing chemotherapy at the time?

Please note, I’m not asking for opinions on what sort of diet to prepare for a cancer patient in a home environment. I’m asking for guidance on where to look for reliable information in this exceptional set of circumstances.

Thank you.

Tags as I wrote them: diet, nutrition, chemotherapy, jail food, cancer patient, food, dietary transition.