I have experienced this sort of abuse. I was not poly then, so I can't say how being poly might have impacted the situation, but surely having more people to love and offer emotional and moral support could not have hurt.
Unless this young woman is in physical danger, she will ultimately have to rescue herself. No one else can really do it for her. You can however, help her arm herself with information, and offer assistance as needed once she makes the decision to leave.
For instance, the Warning Signs of Abusive Relationships (from http://www.recovery-man.com/abusive/abusive_signs.htm
You may be in an abusive relationship if he or she:
Is jealous or possessive toward you.
(Jealousy is the primary symptom of abusive relationships; it is also a core component of Sexual Addictions and Love Addiction.)
Tries to control you by being very bossy or demanding.
Tries to isolate you by demanding you cut off social contacts and friendships.
Is violent and / or loses his or her temper quickly.
Pressures you sexually, demands sexual activities you are not comfortable with.
Abuses drugs or alcohol.
Claims you are responsible for his or her emotional state. (This is a core diagnostic criteria for Codependency.)
Blames you when he or she mistreats you.
Has a history of bad relationships.
Your family and friends have warned you about the person or told you that they are concerned for your safety or emotional well being.
You frequently worry about how he or she will react to things you say or do.
Makes "jokes" that shame, humiliate, demean or embarrass you, weather privately or around family and friends.
Your partner grew up witnessing an abusive parental relationship, and/or was abused as a child.
Your partner "rages" when they feel hurt, shame, fear or loss of control.
Both parties in abusive relationships may develop or progress in drug or alcohol dependence in a (dysfunctional) attempt to cope with the pain.
You leave and then return to your partner repeatedly, against the advice of your friends, family and loved ones.
You have trouble ending the relationship, even though you know inside it's the right thing to do.
If you are in an abusive relationship:
Abusive relationships do not change without sustained therapy specifically targeted toward the abusive relationship patterns. These relationships cannot be changed from one side, it takes mutual honesty, openness and willingness from both parties to work through these issues. Group therapy is highly recommended for abusers, as it helps them to break through the denial that is generally a part of the abusive patterns. (People in denial generally recognize their own dysfunctional behavior in others more easily than in themselves.) This applies to the partners of abusers as well - group helps them to break through the denial by seeing the relationship patterns from a wider view. Certain personality types are more prone to abusive relationships.
If the abuser is unwilling to own their behavior and seek help the prudent course of action is to remove yourself totally from the situation. This is painful, but is generally safer and ultimately better for both parties than allowing the cycle of abuse to continue. Be prepared for the abuse to increase after you leave - stepping out of the cycle enrages the abuser, as it shatters their illusion of control. (75% of women killed by their abusive partners are murdered after they leave.) Learn how to protect and care for yourself. Detachment with love is difficult, but the best solution if your partner is unwilling to work though the issues.
Help is readily available for both parties in abusive relationships. These relationships cannot be changed from one side. Remember that by staying you are condoning and enabling the abuse - and helping your partner to stay sick. If your partner is unwilling to get help the only safe course of action is to totally remove yourself from the situation and seek help on your own.