If you are both minors (even if not), you could try and just talk to an adult you trust. You could say it’s a “friend” or a “conversation you overheard” if you don’t want to go into too many details but it’s good to not let your concern go unsaid, especially if you think she might be in danger
Let her know that it’s not normal. That your parents don’t do that and they give you privacy. Have her other friends tell her that too. Gently. But generally victims are not aware that what’s going on isn’t okay bc the perpetrators are telling them it’s fine and everyone’s dad/mom/etc does this.