Get her in the freedom programme, even just to have a look at the site. He is crying now, pathetic and useless without her, he's vulnerable etc and it's ALL manipulation. Because that's what all of this is about. He's only making himself weak to guilt trip you all to get her back.
Then he will be nice right?
Then he will go back to usual, right?
I'd be interested to know how many of his colleagues, bosses, or mates he's attacked over the years. How often did they need to call the police because of his behaviour?
I'm guessing that it was likely nil.
He CHOOSES to behave this way with your mother.
And your mother is in the vulnerable category right now, however, it sounds like she's one very resilient woman to have put up with his abuse for so long and survived. Humans are afraid of change, but she has dealt with something so hard for so long, that getting away from him will be much easier in comparison. But she needs to realise that he's manipulating her. And that he's never ever loved her, because this isn't and never was love, even if she's always loved him. And that can be so painful to face, that it seems easier to stay.
I hope she manages to get away and live a life like the earlier poster's grandmother. Your DM has lived like this for so long, she definitely deserves some peace, independence and fun.