If someone is hep c positive, why would the hepatologist recommends to put them on a diet?

Perhaps: If the individual has cirrhosis with hepatitis c. Dietary salt restriction and not consuming raw seafood is an optimal recommendation.

Not sure: I can't think of how a diet would matter for hepatitis c (other than an alcohol-free diet). The exception would be if you had cirrhosis due to hepatitis c, in which case low sodium and/or low protein diets are sometimes necessary to avoid the complications of cirrhosis.

Unclear why : I treat patients with hepatitis C. I can't think of any reason why a diet would be used other than for overall improvement in health. Hepatitis c requires very specific therapy. Diet is not one of them