WOW I am suprised how many people suggest going straight to Clink to dob in your FRIEND?? What she is doing is wrong, wrong, wrong but she is your friend (isn't she?). I would talk to my friend about how much trouble she could get into and how unfair it is on those of us who do the right thing. It will catch up with her at some point 100%, but if I was her friend I wouldn't want to be the one to dob her in.
Hopefully if you talk to her she will stop and think about what she is doing. i don't know how she sleeps at night, i'd be a nervous wreck waiting for that phone call or letter (and it will come regardless of if you get involved or not).
Good luck getting to a decision that's right for you!!