Morah Rach – thank you so much for posting. You are right that telling someone to force the girl to stop would be useless but the impression that I got was that people were saying she should tell someone who could work on getting her the help she needs. Whether or not you lose her as a friend is irrelevant. Someone trustworthy needs to be informed so that they can monitor the situation, even from afar, because it is life threatening. And just like with addictions, not knowing they exist often brings people to give the wrong kind of support. Nobody can make her get help before she is ready, but keeping her secret is a dangerous idea. Someone safe and effective needs to be informed.
I once called a pediatrician because I knew he knew the family dynamic well enough to know what to do next. (He couldn’t respond to my information but he was allowed to listen)
Baruch Hashem for your little one, may you be blessed with many more!