Are you saying someone else already booked a portion of her initial request? If so, I think all you can do is offer her an apology and try to explain what happened.
However, if all of the dates in her original request are still available - maybe consider honoring the original price? It's a lot of money, so I can see how it would be easy for them to get angry quickly, even if you weren't really at fault for anything. Things like this will happen from time to time, especially if you are fortunate enough to have your property remain so in demand. I'd chalk it up to a learning experience as a host, an investment in good karma, and and offer the booking to her again at the original price. If you cut people slack like that, you will make plenty of money as a host and feel good doing it!