For the ferals she could buy program flea control for cats. It's a liquid that you have to give with food. Some vets don't carry this though. Here's a link that tells more about it: Program Flea Control For Cats.

I remember that used both program and advantage when I first adopted Storm and my RB Pepper because they both had fleas. If it's a lot less than the advantage then she could also use this after she used the advantage to help keep the fleas under control.