If one serves his employees or his guests terumah [i.e., food tithed as a gift for the kohen; unwitting consumption by an Israelite requires repayment plus a twenty percent penalty], he should pay the principal, and they should pay the twenty percent penalty, according to Rabbi Meir. The Sages say that they [the employees or guests] must pay both principal and penalty, and [the host] must reimburse them for the cost of their meal.
Both the host and the guest have made an error. Although the host appears to be more at fault, it is his guests who have actually eaten the forbidden food. Responsibility is shared between the parties for the violation.