He should write to the airline asking for the cash compensation. He can try the excuses that he only later became aware he was entitled to cash compensation and that signing the voucher forms is binding only if he was advised of all of his rights and alternatives.

He should have been given a free hotel room and some free meals from what you described.

But if he takes the cash compensation required by law, no room and meals are due under current DBC rules but may be due if the passenger is stranded at a connection point.

Travel tips:
http://members.aol.com/ajaynejr/travel.htm