She/he shouldn't have to. Groupon customers should be tipping on what the full amount would've been without the Groupon.
"*Remember: Groupon customers tip on the full amount of the pre-discounted bill (and tip generously). That's why we are the coolest customers out there."
I've got no lost love for GroupOn, but this massive pile on/proof by anecdote thing is not a dignified moment for HN.