This absolutely helps. I shouldn't have put the guest in the drivers seat, but I assumed (cough, bad idea, cough) that AirBNB wouldn't change the price for the nights already booked. The solution was to just decline it, and submit my own with the proper pricing (her original total + 1 additional night).
Why is was an issue:
I had manually set the prices for this weekend higher than my standard rate as it's a college town and it's parents weekend. When the guest submitted the alteration, "smart pricing" changed the rate back to a super low one for all 5 days, giving her a 5 day stay for even less than the 4 days she had previously booked.
Of course, this would have been much easier if alterations had a subtotal breakdown the way all the other pages do. There is apparently no way to get a subtotal breakdown aside from CALLING Airbnb on an alteration (subtotal including: nightly fee, per guest fee, cleaning fee, etc). All that is just lumped together on the alterations page, so there is no explanation of why the price might be lower or higher than what is was previously.