Is she under the care of the local community mental health team. if not she should see her doctor and ask to be referred. If she is, you can phone them and express your concerns. If she is engaging with them, then they won't tell you anything but may try to talk to her.
If your friend is an immediate danger to herself or others,then you can apply for an assessment by an approved social worker(at anytime 24 hrs a day), who could then take the decision to section her.
This is if you are in England... I'm not sure if it applies to other parts of the country but I would think its similar.