A complainant can demand such a copy with acknowledgment and it has to be given under Central Secretariat Rules. If it is not given, he can file RTI Application and can seek such a certified copy under the RTI Act. and such information can not be denied.
basically your relative seeking acknowledgment of his complaint. The best way to get acknowledgment is to give complaint to concerned ACP/DSP of that area. The complaint then go to concerned police station. There you will get the Diary/Acknowledgement number of that complaint.