Under AirNZ’s terms, I am the “Credit Holder”. Credit Holder status can be transferred to other individuals named in the original booking (in this case, I was the sole traveller), but not, it seems, to third parties. From the Ts&Cs:
The Credit Holder is required to use the credit in a manner consistent with the rights of the other passengers of the booking for which the credit has been issued. Subject to this term, the Credit Holder has sole discretion to determine what flights are purchased using the credit, and for whom.
— So it appears I could exercise the credit on behalf of a third party. So we’d have to coordinate: I’d make the booking, you’d reimburse me (via the Sparefare PayPal escrow).