IMO you owe it to yourself to turn her in and you owe it to the other members she had to work with. Clearly she thought she could cheat you..so that's one strike, then she thought she could cheat her classmates..that's two strikes..then she thinks she can cheat a whole list of other people. If you don't put your foot down then she'll be doing this and getting away with it the rest of her life. She deserves what she gets and its only because of luck your able to correct this wrong against you.