Is she capable & knowledgeable enough to buy her ticket by herself? If so, then send the money so she can book and buy it herself. If not, then you can book the ticket online (she can print the booking details or the e-ticket) and make sure you send her an authorisation letter (complete with your signature) and a scanned copy of the debit/credit card you'd use (front and back). She can present that if in case she'd be asked by the airlines for proof.