The thing is, he will have consequences to deal with, they might just take some time. As people get older, it becomes harder and harder to hide who they are. The new girlfriend may be all starry-eyed now, but karma has a way of catching up with people.
I too have an abusive ex, and although he hasn't found a new partner as far as I know, he's living like a glamour bachelor. Gyming it up (he only has the kids every second weekend), flash car, fancy house, lots of new clothes, going out to gigs, travelling around. But I know that behind all the status-flaunting, his house is rented, his car is leased, his business is only being preserved by expenses fraud, he's got a spending addiction, a small dick and ED, and he can't handle being wrong.
It's not a recipe for long-term happiness. It might not catch up with him this year, or next year, or the year after. But the kids are starting to ask pointed questions about why Dad acts the way he does, eventually retirement will loom and he'll not have enough in the bank to see him through, and women in their late 40s or older are well and truly disillusioned enough not to put up with his bullshit for long.
I was 46 when I separated from my ex; this year, just before I turned 48, a lovely man started pursuing me and although I wasn't looking, it has turned into a great relationship. My friend's mum, a sweet and funny but certainly not glamorous lady, met a wonderful man (and a multi-millionaire!) in her 50s, post-divorce. Just keep your eyes focused on your own path and being your best self and leave your ex to his own karma.