What is the ruling on eating food given to my father from KFC restaurants, which the restaurant owners give him in exchange for medicine that he provides to them at a lower price or for free? And is this considered ill-gotten gain?

It is not permissible to sell medicine for less than its price or to dispense it for free, even to a poor person, unless authorized by the proper authority; because that is a betrayal of trust. Your father must repent. If he wishes to help the poor, it should be with his own money or influence.

As for what restaurant owners pay, it is not permissible to accept it because it is a forbidden bribe, unless it is a personal gift that does not lead to favoritism, and it is better to avoid that.

Regarding free meals provided by restaurant workers, if it is with permission or at their own expense, there is no harm. Otherwise, it is a forbidden betrayal of trust that is not permissible to accept.