For many businesses, betting that people will spend a percentage above their discount is a reasonably safe bet. For a coffee shop though, I can't remember the last time I spent more than 20 dollars for a couple cups of coffee and a sandwich over an evening hacking in the corner. For this shop to bet that most people would spend 20+ dollars with 13 of that being the 6 they spent on groupon is insane. If your average sales are $50 or more, than this particular deal might have been viable, but it seemed that many were walking out the door having spent only 6 to groupon for their entire purchase.
I feel for the business, however. I've had to turn down customers that I know aren't going to be repeats, and it is difficult. But at the same time, I would never ever agree to a 75% reduction to my sales. My repetition period for returning customers about 4 years due to the nature of my business, so maybe I don't have the market for that kind of thing though.