Hi I wanted to say thanks to all the Superhosts here who have contributed to this discussion. The guest decided to stick with the booking after being advised by an AB&B Ambassador. Not my preference, as I offered to refund half of the 6 week fee.
My concern now is that I have a guest who is stuck with a booking they didn't want. I can only hope that they have accepted that they have to bear responsibility for the error since they're a two-time AB&B user and simply did not know any better.
However, in response to those of you who urged me to change the listing in so many ways: some of your advice made sense, other suggestions would just be pandering to guests who don't take responsibility for their own errors in judgement. I feel pretty strongly since out of 50-60 bookings, this is the one and only time this type of situation has come up. Which tells me it's an outlier.
As for those urging blanket forgiveness: this guest did raise new issues at the last minute to try and get out of the booking, such as: the location was inconvenient, concerns about accessibility, etc. Completely disingenuous and fairly transparent.
Under the circumstances, I felt it was necessary to establish some boundaries since my experience as a host and an observer of human nature tell me that I have to make very sure that the guest clearly and unambiguously understands what to expect, so I followed up with a long list of just that. I also want them to know that casually cancelling a 5-month old reservation for a six week stay is completely unacceptable. For those of you who advocate caving to the whims of an indecisive guest, I don't play that game. Sorry. Think what you will.
PS: AB&B support for hosts sucks. Period. After hearing that the guest decided to keep the reservation, I inquired about my rights if I decided to cancel anyway. I would end up paying a $600 penalty, the original booking period would be blocked from any new reservations, and I would lose my Superhost status.