My cat ate another cat's vomit, possibly plastic. Is she okay?

Oh no it's not good that Kitty ate that vomit! At most, I would expect that eating this vomit could cause some GI upset, causing her to vomit and/or have diarrhea. It's good that it doesn't look like she has eaten the plastic bag, and if she ate the bag, I would be concerned that the bag could cause a foreign body obstruction. Keep an eye on her for now, and if you see any vomiting or diarrhea, I can give you a home remedy to see if that helps her. I recommend withholding food for 12-24 hours, then feeding a bland diet of boiled white meat chicken (no bones) in small amounts several times a day for a couple of days. Then slowly add back in her normal diet. If the vomiting continues in the absence of food, she vomits the bland diet, the diarrhea continues on the bland diet, she stops drinking water, starts vomiting water, or becomes very lethargic, she will need to see a vet right away. Good luck, I hope this helps!