Renting of vehicles under GST

one of friend's father is engaged in renting of car to particular engineer division. Both are registered under Gst . The problem is they are paying the amount , say ₹30,000 exclusion of Gst . they are saying they will pay the Gst. For this matter what to do.Is this correct under GST law ? .if it is YES what to do by service provider or if it is NO what have to do by service provider .