Have your father been served with summons? Are you sure that a case has been registered against him? Check whether any judgment order has been issued against your father. You should check that first from the county court clerk office. If your father really owes the debt and has defaulted on it, then yes, they can charge interest on the debt.
If a case has really been filed against your father, then I would suggest you to consult a consumer attorney. The attorney can help your father deal with the legal issues in a better way. You can also try to settle the debt out of court. The settlement amount will depend on the debt amount and negotiation skills.