Search Results: why does he lie

My best friend who's a little bit younger than me is in her first relationship, 3 months so far. Things have been pretty awful in it. She's got quite a lot of issues and is living alone at 17, I guess you could say she's a little high maintenance. The boy she is with, 21(not a fan of the age gap either, but it's legal where I live), is quite normal and lives a very everything's handed to him life. He comes down from another state to see her and they met online.
As soon as they started dating it was a train wreck, he was continually picking on her about her mental health problems and doing things to purposefully scare her (switching lights on and off in the pitch black house till the point she cried). İmmediately I said 'f*ck him off' and our other friend and I spoke to him about it because she was scared to. At this point she liked him, he liked her, so she wanted to keep trying in hopes maybe he just didn't understand.
Fast forward to now, the bullying stopped but they argue daily and he yells at her almost everyday he is here about something ridiculous (eg. her asking if he left knife marks on the kitchen bench. He lies saying he didn't then later admits he did and she asks why. Then he starts yelling at her) she's tried out acting call everytime it happens to make sure it wasn't her, and he will still yell at her. He can't take jokes but he expects her to take all of his mean bullying. She's on a dating site right now, which he is aware of, and believes that she could for sure find better.
She's unhappy, feels as if she hates him, they have nothing in common, it's a huge train wreck. I've mostly convinced her to separate with him because it's affecting her, he goes back home in two days and she wanted to break up with him now but she's scared so she might do it when he leaves. she's scared she will regret it, be lonely, and he will either be upset or not care at all and keeps second guessing herself. Does anyone have any advice I could pass onto her? Boys and girls!