I own an agency. A female guest booked with her boyfriend and paid the full amount from her own card. They've now broken up and she wants to cancel. Can we honour her request?

Yes. As a rule, the person who paid has the say over the reservation. However, as the agency you must notify the other guest — in this case the boyfriend — and if he wishes to keep the stay, give him a payment window to pay for it himself, holding the reservation in the meantime.