This is going to be horribly long, so either get comfy or click the back button 
My best friend (lesbian) confided in me last night on a night out, that she has been having an affair for a year with her and her girlfriends roommate. This roommate is somebody she has known since she was 12, and the first woman she was ever attracted to. They do believe they will be happy together, they just don't know how to be properly together.
Her current girlfriend is absolutely horrid to her. A complete male chauvinist in a woman's body. I was at their house when she got home from work last night, and she instantly started asking if her clothes had been washed, demanded that best friend washed something she wanted to wear as it's 'not her role in life' to do such things etc. She verbally and physically abuses her at times, and she showed me finger print bruises on the inside of her arms last night from when she pinned her against a wall for shouting "where are your cigarettes" whilst her gf was on the phone to her mum who doesn't know she smokes.
They haven't slept together in 8 months, she isn't in love with her and doesn't think she ever was, but just happened to 'end up' with her after a previous bad relationship.
Best friend and her roommate have been saving £100 a month each from their wages for when shit hits the fan. She wants to leave current gf, but is terrified of what she will do, as she often threatens her with suicide and once stuck a smashed light bulb in her neck after best friend tried to end it with her. She also believes that she could get very nasty in terms of trying to ruin her in any way she can.
I just don't know what to advise her. She needs to leave her current gf, but how? In terms of practicalities, the threats, the potential backlash of ending it... how?
She is coming for a coffee through the week to tell me more about it as there's a lot she couldn't say last night. I'd like to show her the replies. Maybe seeing that she can do it in black and white will sink in more than somebody saying "It'll all be okay in the end".
If you have made it this far, I applaud you, and thank you for any advice :)