facebook does remove the exif. You could ask her to send you an original (in a tactful way of course), and check it there, then verify by asking her what kind of camera she used, if she says something different than what the exif says, well....

Personally, and depending of course on circumstance, I would confront her about it. Tactfully, but, if there are no other possible suspects, you can state something in a way that shows you know, at the very least it could make her feel awkward or guilty or something, maybe she'll even give it back.