Little bit confusing!!!!!!!!!
If your relative is a pakistani diplomat serving in another country, he/she can bring vehicle to pakistan under the same rules which applies to all overseas pakistanis (TR, PB & Gift). There is no duty free thing for pakistani diplomats serving in another country.
Perhapes you are mixing it with other country's diplomats serving in pakistan. For example a UK diplomat working in pakistan can import a duty free vehicle and even for that they need permission from foreign office and get a duty exemption certificate from CBR.