I'd say 185's about right for her with proper form. She's not touching her chest, and she's also totally lifting her ass off the bench, which makes it way easier too. If she had her ass and shoulders planted, didn't move her feet, and touched her chest, she'd be good for about 185. That's pretty fkn good.
The arch is to reduce the distance you have to press, but it's more than that. Shoulders, ass, and feet can't move, so part of the trick to good heavy benching is to set yourself up in such a way that your body is angled for maximum drive. That includes getting your feet under you and arching enough that you are actually using your legs and back when you drive the bar up from your chest. Benching is a full body press if you do it right.