It sounds like this will be resolved to your satisfaction, so that's a good thing. Getting your photography out and about is important and with that goes the risk that someone may use a photo without permission or compensation. However, the alternative is not to have your photography seen at all and no sales. I would suggest that you try to make the Groupon use of your photo work in your favor, I would price the use fairly, not punitively, and then suggest that perhaps they might be interested in licensing some of your other images in the future. Turn this fellow into a new client, if possible!