The mother is the key to this. No three year old, unless there are SEN issues, can present behaviour so challenging that the mother is manipulated by it. Let's be clear here, your friend chose this path, possibly through laziness, or a misplaced sense of 'anything for a quiet life.' It is easy to give into kids, much harder to stand firm. Toddlers push against boundaries, but they learn that they won't always get their own way however much they scream and kick off.
This 14 year old still has the toddler mentality of a three year old. Her mother's lack of parenting and boundaries meant that she's kicked against every boundary the school has set her.
Then along comes SF, and of course, she kicks off again.
Her mother had reaped what she's sown. Her mother needs to take responsibility here, rather than passing the buck. She's way too passive in all of this and always has been. Her daughter, her boundaries. The SF then supports her in these boundaries and together they present a united front. Your friend must stop hiding behind her husband, take control with her daughter. She should contact her, tell her she has a home and is loved, and set out her boundaries.
Young kids need boundaries and structure and after they've pushed them, they usually realise that they're there for their own good. They make them feel secure and valued. "I love you enough to say no" is the message they understand. This troubled teen has never felt this security, and that is your friend's fault.
The submissive little woman act will not earn her daughter's respect. Whether she is naturally strong or weak, she must step up and sort this. The buck stops with her.