I think it's rarer for cats to have ACL injuries (at least, I've heard of only 2 or 3 cases), but it's pretty common in dogs. With a dog, the recovery usually entails making the dog take it easy, NOT putting ANY weight on his leg for at least a month, and then you take it sloooooooooow. My dog tore his ACL when he was 7, had surgery, and I took things so slow and cautiously with him. When he went up or down stairs, I had a towel around his belly and lifted him up so he couldn't so much as touch the stairs with his back legs. For the first month, he was crated and was only allowed out to poop and pee (the towel went on during those, and I quickly mastered the art of moving the towel away at the right time so it wouldn't get splashed with pee). After a month, I started letting him walk farther than the "Pee Tree" (the tree he'd go to pee against before we headed back to the house), and then I'd take him to the end of the street and back. After a month, I allowed him to start going for walks again, without the towel, but we started from scratch again. We'd go to the end of the street and back, without the towel, and gradually increase the distance. He started at a walk and was only allowed to start jogging again when he could prove to me that he wouldn't limpy-skip. When he was allowed to start jogging, he wasn't allowed to go faster if he showed me the limpy-skip. Basically, I just took it all slow and I didn't allow him to even run for at least 3 months. 4 months later, he was pretty much "cured", and after 5 months, there was no question about it. Now, he doesn't even limp, even in cold weather. Well, maybe once or twice during winter, he'll wake up from a deep nap and have stiff hips, but he's also 9 years old now, so some of it's arthritis.
With a cat, I think it'd be pretty much the same stuff: try to prevent her from overexerting herself, and especially keep her from jumping around if you can. Last thing you want is for her to overextend her knee again and ruin the good surgery did for her. She may have lost weight due to pain, personally. Did she gets meds? Jake had some pain after surgery and he got some Medi-cam (that was delightful to give to him, lol) and some antibiotics (and when the incision got infected and puckered, he got more antibiotics! lol). Jake didn't lose weight (unfortunately), but it did take a week for him to have a bowel movement. Apparantly surgery can do that, can make the bowels slow down for a little while.