My girlfriends mom hurts her (mentally and physically)?

My gf and I are in a long distance relationship and constantly on the phone or while were on skype I hear my girlfriends mom Screaming, yelling and crying at the top of her lungs at her. My gf is one of the kindest and most loving people I have ever met. We have been together for 2 years. I am 23 while she is still in university. We are long distance and when she's off for the summer or breaks she has to stay at home. She says she hates it there and cries most nights about how her mom treats her. (btw its just her mom and my gf in the house). She says her mom is Bipolar and I believe her. Where I am looking for opinions on is just the other night we were on skype, my gf was in her room crying while her mom screamed through the house at her. Yelling for no reason at all (this is common). I tell her to not take it to heart, that it is the disorder yelling and saying those things; and just focus on our future together a place away from all of this. Her mom then came in her room and I saw my gf defending herself from her mom. Her mom was trying to do something. I couldn't tell by how the laptop laid on her bed but I could see my gf trying to hold her moms hands away from her. Shortly after, my gf turned around and shut the laptop. I don't want to think anything would ever happen to where her mom would hurt her but since I am not there (700 miles away) I can't protect her or have her come live with me. I am afraid if I call the police (unanimously) that it will cause so much emotional stir up that it will cause more unrest when the police leave. And that they will eventually figure it out that I called and reported abuse. I am at a loss. This happens about once every 1-3 months. No one should have to live in a house like that. They need a home where they are loved. -Thank you all for your help in advance.