I think there's a few reasons to that.
Reason 1: As her friend mentioned, when it comes to herself, she can't face the truth until it's looking her dead in the eyes. She has a LOT of empathy, and b/c of this, she is easily manipulated.
Reason 2: She still loves him to the point where she can't leave him even though he's mentally and at a point, was physically abusive. This is a common & very difficult matter that most people struggle to over-come in a toxic/abusive relationship.
Reason 3: Getting a divorce seems to often be looked down upon in their culture and could "damage" her reputation in the future (both work, and social wise)
Reason 4: She's scared. Think about it. She loved her "faithful" boyfriend and took precautions not to get pregnant, but her whole world did a complete 180 on her and now her whole life is upside down.
She's pregnant with a baby that she WANTS to keep (whether she's with the father or not), and her best friend keeps telling her to abort it.
Her now husband isn't providing any reassurance that he won't cheat again. It was mentioned that she doesn't really talk to any of her relatives so if she needs back up or help, she doesn't really have anyone to go to.
At this point she's basically a young single mom, that has no idea what she can do or how she can provide for herself and the baby.
She's in a very difficult situation and the option of simply divorcing him (if the dude agrees to) put's her in just as bad of a setting as she's in now. (Actually an even worse one as one of her main reasons for wanting to marry him is b/c she realized she currently can't take care of the baby by herself).
And with how long she's been pregnant she's still in the danger-zone for a miscarriage. I'm amazed the girl hasn't been having panic attacks
poor girl :'(
dude wtf WHY THE FUCK YOU STILL WITH HIM?? LIKE JUST HANG HIM DIVORCE PAPERS UUUUGGGGGHHHHHHH