Here is the situation. My employer hired a person who had cancer and is in remission. He won't hire this person full time because he doesn't want to give benefits. If the cancer came back, it would cause company health insurance premiums to go up.

Is there such a thing as medical discrimination?

Technically, no. There is disability discrimination, but I don't think this would pass that bar.

However, if he is keeping this employee on a part time status SOLELY for the purpose of preventing him from having access to the company health insurance policy, that is a violation of ERISA. The employee would then file a complaint with the US DOL.