What are some signs that your best friend's boyfriend is manipulating her?

Anonymous

My best friend has been given the ultimatum to cut me and her other friends off because we don't like her boyfriend's behaviour towards her. They've only been together 1 1/2 years. It started off well but in the past year it got violent. He grabbed her arm and he's hit her quite a few times when he doesn't get what he wants. He seems to control her behaviour by checking her phone to see if she's contacting us or anyone he doesn't want her talking to. Every time she goes out she accuses her of flirting with other guys and gets violent with his jealousy.

She's cut everyone off now except for occasional small talk (which her boyfriend still doesn't like) as she truly believes that we've treated her boyfriend badly. He has persuaded her that we aren't her friends, that he's her only real friend. It's because I stood up to him hitting her. I'm really worried for her. What are some signs that someone is being manipulated in a relationship?

Updates
3 mo
If I could give all of you MHO, I would. Thank you everyone who answered.
What are some signs that your best friend's boyfriend is manipulating her?
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