The XY forum seems most appropriate for it, because it's XY exclusive.
Edit: It doesn't sound like an advertisement to me. She's not trying to advertise it for GameStop, just discuss it. That being said though, you might be better off just asking an XY mod, since they're the most knowledgeable on their forum and it'd really be up to them as to whether or not it's allowed. But in my opinion, it doesn't seem like an advertisement and I'd let it in if it were up to my discretion.
Edit: It doesn't sound like an advertisement to me. She's not trying to advertise it for GameStop, just discuss it. That being said though, you might be better off just asking an XY mod, since they're the most knowledgeable on their forum and it'd really be up to them as to whether or not it's allowed. But in my opinion, it doesn't seem like an advertisement and I'd let it in if it were up to my discretion.