As a former Animal Control Field Officer and long time State Pet & Wildlife Rescue Agent I can tell you that I have seen dogs and cats eat allot of things. As long as your cat swallowed the foil and is not choking on it...she doesn't need to go to the Vet now. She should be able to pass it as the intestines will ball it up. I have seen this many times before, especially when pets like to eat Hershey's kisses with the wrappers still on.
If your kitty does not pass the foil in the net 24-48 hours...She should be seen by the Vet as it could cause a blockage or get lodged in the system. But as long as she isn't choking on it now...It should be able to wait.
Let me know if you need more information on this as I will be online all morning...