This is precisely the reason I didn't take advantage of that offer, even though I could have. When you pay with a GC, you are at the mercy of the company. When you pay be credit card, you have the recourse of your bank's dispute process - which is there for precisely this sort of eventuality (e.g., a travel provider cancels your booking and refuses to refund your money). I think your partner's only recourse is to threaten to sue AirBnB.