Immoral?? Maybe. Fraud?? Initially not. Theft?? Not. Unethical?? Quite likely. Illegal?? Definately Not!!!!

Had he made an attempt to alter or misrepresent his voucher when booking the ticket(s) it may have been fraudulent, if he actually uses the ticket it will be somewhat fraudulent. It definately was not illegal although it almost probably would contravene the carriers terms.

Immoral and unethical??? Well that is between him and his conscience.

If they catch on will he have to pay the fare or forfeit his FF account. Very very likely.