It should count because it's written in the MBG policy that if you open in one, you can't then use the other. At least by going through PayPal, she has to pay the return postage, so she's the one who's going to lose out in the end. I wonder if she thought of that before opening that dispute? She would have been far better off leaving it with the eBay dispute. 

Have you left feedback for her? If not, wait until you get the item back, then leave feedback mentioning that a refund was issued. Something along the lines of "refund issued <user ID>. Thank you". That way it's a positive comment and everyone else will be able to see who it is. Other sellers in the future will also be able to see that she has been given a refund and see if there are any patterns forming when she tries to buy off them.