Poor woman.
I think it's easy when you're the one looking in, judging the situation. But, I can totally see your frustration.
There's a multitude of reasons why she might have gone back.
She might be scared, he may have threatened her.
He might be love bombing her, promising he's changed.
She might feel like she can change him.
She might not feel strong enough on her own, after years of being worn down.
She might feel like she has no other choice.
She is probably making excuses for his behaviour and more than likely blaming herself.
She's not the bad one for going back to him.
In most dv relationships, the woman will attempt to leave approx. 7 times before eventually leaving.
Women are at a higher risk of being murdered after leaving an abusive relationship.
At this point. Most of her friends and family will be turning their backs on her, playing into the abusers hands.
He will end up being the only one she has.