I'm not 100% sure whether this is a genuine Employee benefit or a Marketing ploy.

My Employers run a very similar scheme which is indeed primarily to generate extra business - We are encouraged to send out the mail & voucher to as many people as possible (& they are then in turn encouraged to pass it on).

I've looked at the T&C & there doesn't look anything too obvious to me that indicates you have to provide clear proof that you have check-able links with the originator of the mail.

I'm thinking if they were that bothered about checking the details you would be asked to provide more information as part of the booking process.

I would also expect a big "I understand I need my Friends & Family status to be validated for this booking/rate to be honoured" check-box somewhere during the booking process.