If you sell anything that you buy, you'd need to have a 77.5% profit. Most restaurants only do a 10 to 20% benefit usually.
They'd be better served by pinning a huge promotion on their front like :'50% percent on friday'; they'd reach more customers in they neighbourhood, who would be more likely to come back and they'd keep a much better share of the deal.
It's not even nice for your actual customers who come the same day and pay the full price just to discover that they got screwed because they could have paid half by going to groupon.