I in no way support a buyer keeping a bag, counterfeit or not, and being reimbursed. That is too open for fraud. AFAIK she will have to return the bag for a refund. Yes, the seller should be responsible for return shipping, but they are not required to. Personally, I would pay the postage just to get my refund and be done with it. Appropriate feedback can be left at that time to warn other buyers.
It is ok for a regular transaction but it is an international transaction 18 pounds to ship first and then pay again to send back a fake??
Not in my book. And I won't even go into the legality of it all.
The least the seller can do is take care of the postage.