This happened to me. I host AirBnB's in San Diego and someone booked a year out for Comic Con right after the last one finished. I never had that far out a booking before and hadn't set pricing for the following year in my pricing tool yet. I let him know I wouldn't be able to honor it and he understood it was a too good to be true deal. He asked me what my rate would be and I wound up actually meeting him halfway.
I don't think it's necessarily a dirtbag move on the host's part what happened to your friend and I have a feeling your friend knew to an extent that they found a deal that seemed a little too good to be true themselves. There aren't that many businesses that honor price mistakes and these things do happen.
What matters a lot here is how long the reservation is held in my opinion. How long a window was there between booking and being notified of the mistake? I think the 30 days the host allowed the reservation to stand is too long and I would have likely eaten the mistake.
In the same vein this occurs with guests when they make a simple mistake within my cancellation policy that won't entitle them to a full refund. I always look at how much time has lapsed. If they held my property up for being booked for a week or longer I'm a lot less inclined to refund them the rest of their reservation. If it's the next day it's a lot easier.
FWIW AirBnB has recently changed it host cancellation punishment at least for superhosts, not sure about non-superhosts to where we're now allowed to cancel up to 1% of our bookings per year before losing superhost status. A welcome change imo.