Thanks very much for your responses. I think you guys have confirmed my worry, which is that anything I directly say is going to perceived as nagging, and worse it could even have the opposite effect that I'm hoping for.
I know that she won't quit unless she's ready to. It's frustrating for me because I feel like I can't really do anything to help, other than wait. @cassie's suggestion is a good one, although I'm closer to the fiancee than friend role...
@occasional01, so to be sure I understand, you're saying that her only choice is probably to quit cold-turkey, rather than try to continue with a reasonable dose?
How do you guys think I should treat the subject of the abuse? Should I just accept it and support her, no matter how much she wishes to do? (obviously I'm not going to encourage her or enable her, but I won't complain about it). Or, should I tell her I think she's doing too much??