Don't get her a bike that is too big and figure its fine because she's just riding casual....

Size is sooooo much more than stand over and more than about mounts and dismounts. It's about power and comfort. On a bike that is too tall / long she'll likely just be frustrated, feel weak, slow and her body will hurt. While it may be fine and not noticeable for short jaunts, it certainly won't make her more likely to want to be more serious about riding.