Why do women 'go back'

[Old Aussie]

I realise this a very complex issue, often with many factors contributing.  And I'm not trying to insinuate that all women "go back".

I am trying to support a friend.  Yes we're just friends, but I'm trying to wrap my head around her behaviour.

He last two relationships have been extremely toxic.
She seems extremely angry with her ex, and accuses him of owing her a huge sum of money.  Yet she has given up any change of recovering any of the money, and at times seems genuinely scared of the guy.  Anytime they communicate, he ends up being insulting and verbally abusive (either via phone or text.)

She's told me that she is completely over the guy, and has been for over a year, and that she want's nothing more to do with him.
 

But this is the part I can't get my head around:  She keeps re-opening communication with him.  
It always ends the same way, him being nasty and abusive, her in tears and on the verge of a break down.  Yet she keeps doing it.

She will have him blocked on all social media, and blocked all his phone numbers.
But then she will unblock him, and start up communication again.  Ostensibly this is to ask him to repay some of the money, etc, but she already knows what the result will be.  

Plus (and I know this sounds unkind) she opens herself up for more of the same.  Once she has re-established contact, she'll message him something like "if you ever really loved me or cared for me, you'll pay me back some of the money".  And of course that just draws the same nasty shithead responses she's always gotten.

End of the day, I'm not trying to "save her," nor "fix" her life.  I'm a friend, and I'll support her, but I would like to try and understand this behaviour.