What I learned in food service is that a general formula is 30% food cost, 30% labor, 30% overhead, 10% profit. If things are running well, you take home some money at the end of the day. The margin for error isn't huge.
So assuming that these customers didn't buy anything over $13, her COGS should have been $13 * 0.3 = $3.90. This also ignores the fact that there are people who paid $6 for the coupon and never redeemed it.
Now lets assume that some people didn't redeem the coupon and that her COGS is a bit lower, so COGS = revenue. As labor and overhead are 'fixed', she shouldn't have lost any money as long as these weren't existing customers who decided to use the coupon.
If 100% of her existing customers used the coupon she certainly would have seen her profits hurt. If 100% of these people were new to the business, she wouldn't have made/lost any money on each transaction, but she would have obtained a lot more exposure.
So before deciding to do one of these deals a business owner needs to have a clear handle on their costs, the expected mix of new/existing customers, and the expected revenue from the new customers over some reasonable length of time.
Now lets assume that some people didn't redeem the coupon and that her COGS is a bit lower, so COGS = revenue. As labor and overhead are 'fixed', she shouldn't have lost any money as long as these weren't existing customers who decided to use the coupon.
If 100% of her existing customers used the coupon she certainly would have seen her profits hurt. If 100% of these people were new to the business, she wouldn't have made/lost any money on each transaction, but she would have obtained a lot more exposure.
So before deciding to do one of these deals a business owner needs to have a clear handle on their costs, the expected mix of new/existing customers, and the expected revenue from the new customers over some reasonable length of time.