You can request the information through the ND Open Records laws, but, the agency may deny you claiming they contain investigative information that is exempt from disclosure per N.D.C.C. § 44-04-18.7. If the agency denies the request, you or your friend may be forced to hire an attorney to sue for the record if that is something any of you are willing to spend a few thousand dollars on trying.
There may also be a legitimate reason to keep the caller anonymous. In fact, it's entirely possible that the call IS anonymous because they did not provide a name and it came in on an unrecorded line.
If your friend is going to court for some reason, and the call is a key part of any probable cause used for a detention, search, or arrest, your friend's attorney will know how to seek the info through discovery or subpoena.
Frankly, you can pretty much bet that the agency will seek to deny the request if they can in an attempt to protect the reporting party from retaliation. Most agencies will not freely give that info up if they are not required to do so.

