Why I won't be using Groupon again.. A consumer perspective. 
I think you might be overestimating how much insight the waitress has into the business.  It may be the case that this was a close, family-held restaurant and they have a daily pow-wow on cost of goods and marketing strategy, but I wouldn't bet that way.  My guess?  Groupon customers are poor tippers, either from adverse selection or because they either a) attempt to tip with leftover funny-money or b) tip based on the amount they paid rather than the face value.  Thus, mentioning you to other waitstaff: they're complaining about prospective tips.  ("Gah, I drew another Groupon while you get a good table.")Put it this way: if you had put twenty quid in her hand when you sat down she would probably have instantly become the most devoted Groupon fan in the world.