It is highly unethical to request for people's NIP numbers without their consent.
NIP numbers are known to be liken to Social Security or personal identification numbers, are extremely sensitive and confidential. So, It is unethical to share information of this nature due to its sensitive nature and potential for misuse.
Sharing or seeking such information without proper authorization is inappropriate and violates ethical standards. One have to keep their privacy secret and adhering to ethical guidelines is paramount when seeking personal information or contacting professionals.
So, the proper channel is to ask the person if he or she is willing to share it with you