If you're right next to her in line, it will be difficult to blow the whistle right there and act as coupon cop. If you're 2 or 3 persons behind her, you can look for the MOD and speak to him/her discreetly, that will be the better approach without embarrassing the person.
If she has a binder, it means she has read enough to make one of those and collect all coupons in it. IF we give her the benefit of the doubt that she honestly doesn't know, when the MOD tells her, she will likely be very apologetic, while those who have been doing it all the time become hostile and possibly create a scene @ the store
I know some stores would not apprehend shoplifters caught in the act, such act may be comparable with shoplifting using coupons but if they have a policy to let them go, nothing we can do about it.
I know most store management staff are disappointed about this policy but their hands are tied and it is ironic that it is the store staff that get reprimanded by corp once these people complain