"I mailed this book (Wolves of the Calla) but forgot to mark it mailed before the deadline, and it was canceled. If you have an active request on your account page for Wolves of the Calla it is not with me--it is with a new sender. Your request must have been passed on to that person, and you will get 2 copies of this book if the new request is not canceled. If you do have an active request on your account page for this book can you cancel it? You will be getting a copy of the book from me. You can then mark my book received when it arrives, from your Transaction Archive. If you are willing to do this I really appreciate it! Just let me know if you have a new transaction on your account page, and if you are able/willing to cancel. I am very sorry about the inconvenience. I am going to mark my books mailed before the deadline from now on!"

It was a WL book and I just went back into the WL queue, so I don't have another order pending. That part is ok.

What I'm not sure of is how to have the records reflect that she did in fact send the book to me, since there's no record of it. How will I mark it received with or without a problem? Or do I just buddy her a credit and let her worry about the bad mark on her record? I'm also not sure how to handle this if there's a problem with the book when it arrives.

When you receive her copy of the book, you can mark it received from your Transaction Archive (under your My Account tab). 

In the meantime, you could do this - add a requestor condition saying something like "If this is for Wolves of the Calla, please say it doesn't meet my conditions" - that way the person posting it will be able to send it to the next person in line, and if for some reason the book  never gets to you, you can remove the conditions and keep your place in line.