advice very appreciated

my best friend who is practically a sister to me, is in a relationship and her boyfriend shows signs of abuse. we have been friends for nearly half our lives, i’m almost 17 and she just turned 16 in april, her bf will be 20 this year. we hung out to the point that she pretty much lived with me. her bf has her passwords and goes through our conversations, throws fits to try to manipulate her into getting *things* that he wants, he will hit animals with his truck for fun

this is the second time she has gotten with a guy like this and the last it resulted in us not being friends for a bit because she would lie about him, which is what she is starting to do again. i worry for her since she’s been withdrawn and hasn’t actually hung out with me in 3 months. when she came to get clothes that she had here, her bf came in to my house with her and didn’t say a word to me, he just watched. him and i used to be friends which seemed odd to me, he also threatened to “ beat the shit “ out of the dog that was jumping on him, which was her dog but she left her at my house and has seen her twice in almost 3-4 months. when he wanted to leave, he seemed to coax her into leaving even tho she expressed that she didn’t want to leave yet. i really don’t know what to do but i know that it might not be bad rn but it will escalate. her mom loves her bf & her moms bf is similar so she sees no problem with the things she knows about.