I had to look close, but I can see the halo.
It might be a shadow on the background, but it may also be the result of a quick edit where they wanted to edit the background or subject separately...but rather than make a careful (and time consuming) selection, it was done quickly.
Part of the problem, is that it's hard to edit in and around all the fly-away strands of hair around her head. If I were editing this as a final version of a portrait photo, I'd probably get rid of the fly-aways, which would then make it easier to edit the background if needed.