You're concerned for her safety because her dad is an abusive arse. That has nothing to do with his new GF.
If your only concern about her is that he met her online, well sorry, that's how a lot of people date these days.
We're not all nut jobs, most of us are just busy people who don't have the time or opportunity to go and hang around in a bar to wait until we're picked up, we prefer to find out about a guy from some basic info on a website and then meet him. If we get on well, it may become a relationship, which apparently your ex & his GF have done. That doesn't make her a loser or a weirdo.
The fact that she has no children of her own is also irrelevant. If you're worried she will try and take your place, please don't. It's not as easy to love someone else's children as it is your own. She may well be fond of your daughter but I doubt she will be trying to take your place in any way. She'll probably look forward to the times she can be with her BF without DD in tow.
If you have genuine concerns about her safety then address those with the relevant authorities, but don't make this about the GF. I dreaded meeting my boyfriend's ex, but when I did we said a quick hello and she went off muttering about me seeming nice. She knows nothing about me from our brief encounters, but she trusts that my BF wouldn't allow someone to harm his kids. My ex doesn't have a GF but again, if he met someone it would be one of my business, especially if the kids liked her.