If the place has closed down as in gone bust, there is unlikely to be any refund due, because places close down because they run out of money and there is a set order in which any available funds are distributed, with HMRC, the likes of the council for business rates and any secured creditors at the front of the queue.
After all that, there is rarely any money left for any other creditors such as gift voucher holders.
However, if you paid for the vouchers by credit card and they were for more than £100, you can get a refund from your credit card, and obviously it's the holder of the card who needs to organise this. If you paid by debit card, you might be able to get a chargeback, but assuming that there has to be some money available for the bank to take back, I don't know if this is possible, but worth a try.
www.moneysavingexpert.com/shopping/section75-protect-your-purchases/
Of course, if you do manage to get money back via this route, you would probably feel obliged to buy the PILs another gift, as they've lost out up to now.
But to be honest, I'd advise everyone to steer clear of gift vouchers, because there's likely to be a lot of companies go bust over the next couple of years, and in this case, you often might as well set fire to the money.
Just give cash, or agree to stick to exchanging token presents only, much easier and less risky.