Unsupervised 15 year old

What would you do?
My step children live interstate with their mother who has always been a difficult person to deal with. She is quite narcissistic and has used emotional manipulation for some time to control the children’s behaviour. At times this has caused my partner to have no contact.
The only time this improves is when she has a new partner as she then is no longer interested in her children. While this is great for my partner’s relationship with his kids, it’s obviously an issue for the kids.
She is currently spending the majority of her time away from home, staying up to 5 nights a week with her new partner while her 15 year old daughter is left at home alone. Her brother is older and sometimes there also but they have a strained relationship. The mother comes home in the morning to take her daughter to school. We only know this because his daughter has told us. The daughter has been told to not tell us and knows that if her mother was to find out that she told us all hell will break loose.
I don’t know if we are overreacting however we don’t feel comfortable with this and are sick with worry.
Here is the dilemma - raise the concern with the mother and have the daughter deal with the repercussions of her mothers wrath … or let it continue and offer support from the sidelines. It’s so hard to know what to do when both options are heart breaking.