It's the law of the land... In his jurisdiction, if you get screwed on the price, you're entitled by law to 3x the amount you were being screwed out of. He wasn't asking for anything special, just what the law dictates. Also, restaurants BETTER keep tabs on their online pricing. Everybody and their grandmother uses the web to check prices/availability and even do ordering. Especially true for restaurants offering takeout/delivery. It's not the 80's any more. Welcome to the 21st century.. Actually a properly managed website will have instant pricing updates and it's not all that expensive to set up... Cheaper than a couple of manual corrections, so the restaurant doesn't have an excuse not to have proper pricing advertised. Otherwise, it's bait and switch.
BTW I personally wouldn't do what he did, but I respect his right to demand his due.