I worked with an orthopedic surgeon for almost 2 years. Please be sure to have all the pre anesthesia bloodwork done, and say yes to pain management. Do you trust the vet doing the surgery? It's not a simple procedure like a spay or neuter. There are old and new ways of correcting a torn ACL. I would ask how many he or she has done. Just my opinion and you know what they say about that term. There is also a chance of it happening again. This kitty needs to loose some weight if she is indeed 13 lbs. She will most likely come home in a huge bandage type cast which in time she will learn how to maneuver with.
Here is a link to where I used to work. Maybe there are some links from that page, or some helpful information. http://www.vss.org/
Good luck to you both, Denise