Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about your girl ate a piece of a leaf from a Peace Lily.
These are not true lilies in that they are not truly toxic and don't cause acute (sudden) kidney
failure like true lilies do. They can be very irritating to the gastrointestinal tract and can lead to erosions and thus not wanting to eat but if a small amount is eaten or they drink or eat afterwards they may flush out the irritants. In some cases it may take a little time to see symptoms but I would think if has been more than a couple hours and she is showing no symptoms then she simply may not have eaten enough to cause her any trouble. Even if she does show symptoms they usually come along with time and fluids and gastrointestinal protectants like sucralfate.
If she chewed on but didn't really eat the plant then she may have such minor irritation that she isn't showing any symptoms.
If she feels well otherwise I think you can try treating her at home.
I'd give her pepcid-ac (famotidine) at 1/4 of a 10mg tablet per 5 to 10 pound cat, 1/2 of a 10mg tablet per 10 to 20 pound cat every 12 hours. This will reduce stomach acidity and make her more comfortable just in case she does have mild irritation. I would use this for at least 2-3 days and if she shows any symptoms (drooling, reluctance to eat) then I would give it for 2 weeks to allow her to fully heal.
If she seems at all nauseous or uncomfortable you can feed a bland diet of 2/3 boiled chicken and 1/3 boiled white rice with some low salt chicken broth added to make it soupy is an easy to digest and soothing diet. I would feed her this for 4 to 5 days, then gradually start mixing in her own canned food
, taking at least 7 to 10 days to convert her back to her regular diet. The longer you can feed a soft diet the better she will heal. Any further irritation will slow things down.
If she vomits, refuses to eat or is drooling excessively after the first 48 hours then she really needs a look by a veterinarian.
Let me know if you have any further questions.