If the facts are as clear and straightforward as you say, then your friend has nothing to worry about. He sold the tickets in good faith; at the time he sold them he did not know the gig was going to be cancelled and re-scheduled (if he had known, the purchaser could have a valid claim for the return of her money on the basis of misrepresentation). In any event, the purchaser won't suffer a financial loss as the gig is going to be re-scheduled (if it's cancelled altogether, then she, along with all other ticketholders, must look to the concert promoters for a refund).