Based on the author's blog post, the split was determined beforehand, but Groupon did not allow them to set an upper bound on coupons sold. This seems to be a recurring story - business owners willing to take a loss for marketing via Groupon, but with no effective ways of controlling an upper cap for this loss.

The part that really tuned my brain anti-Groupon in this article was: Groupon sold consumers a $13 Posie’s credit for $6, and then sought to keep the entire $6.

I mean, that's a 100% cut with the business getting nothing. If that's serious, Groupon look unethical; if it's not, then the coffeeshop is lying (and others have doubted the sincerity of the figures, too, as well as their ability to run a business full stop). Either way my mind is slightly twisted to dislike Groupon now. Subtle!