Hi The Fox & the Hound, no I don't think you're overreacting and I'd say that it's really good that you care so much. But other than telling her what you know about him there may not be a lot more you can do other than to be there for her.
Although you might want to drop in your concerns and some advise along the way but maybe do it in a non-confrontational, non-pressurising way in case she goes on the defensive. And that may make her route for him even more.
You could maybe try boosting her self-esteem and confidence as well though as they might be a bit lacking if she's hooking up with this guy. Maybe there's stuff she might talk about that's happened to her that's leading her to be so "desperate" or choose someone like him that you could help her with??
And maybe try to open her eyes a little to the good guys out there??
But kind of judge from her reactions what's getting through/working and what's not.
But if at the end of the day all you can do is be there for her then that's massive!!! And kudos to you for caring so much!!!
Alison