<human>: Give me a shortened version of the text

Here's the story. My friend's name is Greg. He's been my best friend ever since third grade, so he's almost like a brother to me. Well, he met this girl just over a year ago and asked her out, to which she happily accepted. Her name is Emily. They have been dating for (as I mentioned) just over a year now. Emily is a great person, don't get me wrong. She set me up with her best friend, Rebecca, and we've been dating ever since (very, very happily, seeing as she's not possessive). 

The thing is, and this is what confuses the fuck out of me, is that ever since Rebecca and I started dating, Emily has been progressing into a psychotic possessive bitch. I've only ever called two girls a bitch in my entire life. This is one of those cases. 

She ranges from stealing his Macbook and 3DS for her own enjoyment (she takes them overnight to her house whether he likes it nor not), to not letting his best friend of eight years hang out with him (Oh yeah, that's me), to going against not only her parents' judgement, but even Greg's judgement so they can be together. She once logged into his Facebook and deleted everybody she doesn't "like". My friend is more whipped than the cows off Willy Wonka and the Chocolate Factory (whipped cream; ba-dum-st). I asked him why he puts up with it, and he said "I hate it when she gets pissy". He's just proving that she can get whatever she wants by acting like a three year old and pouting. Oh, and I've seen all of this first-hand (when she "allows" me to hang out with him). Everyone close to him has already told him that he's being controlled like a midi instrument in Ableton, but he still goes along with it. I'm sick and tired of seeing him suffer like this, Reddit. What can I do? What advice can I give him? I'd prefer not to have his relationship end, and I'd prefer to keep our friendship, but if it comes down to it I'm prepared for the worst.
<bot>: My best friend's girlfriend is possessive as hell. He knows this, but doesn't know what to do other than put up with it and exist in misery. What do?