Mira Sorvino's speech *SPOILERS*
First let me say, I like this film very much; The speech that Mira Sorvino gives about the responsibility of everyone else was very accurate, to the point, and important to take in.. Yes, I agree others should have at least mentioned the abuse to Sarah's mom, but Sarah DID have a responsibility to end the relationship early on, before Rob became possessive. Even if you love someone, its SO important to end things if they even HINT at being abusive, emotionally or physically. There is usually a time where the person will back off, and not become violent and/or possessive like in this film. No one deserves to be abused, and it is not their fault. But here is the reason they may stay with the person who abuses them:
the person has low self-esteem, and think they cannot get anyone else.
the person thinks they can 'change' the abuser. You cannot change the abuser.
The person thinks it's their fault, maybe they didn't do something right. Even if they did do something 'wrong', that doesn't justify hurting anyone.
The abuser is psychologically unstable. Nothing you do will change them -- as this movie shows, it can end up in death.
Even if you will lose your friend by telling someone, that's better than losing them for good.