First off, if she sent it to Gamestop she better have had their extended warranty. If not, that was a mistake. Secondly, the worst Gamestop can do is send it back without repairing it and then ban you from the store for knowledge of your possible piracy. (Doesn't matter if you scam them or not to get your games, just seeing a flash cart is grounds to be banned. I've seen it happen before.) If they keep the cart and/or dispose of it, then you have a legal case. They disposed of your property without your consent. Now, if you pursue said legal issue, you may be immediately followed by one for piracy. You'd get your cart and DS back, but you'd lose them in the next case.
Basically, hope for the best. If they do their job right, you won't get your repairs but you will get your stuff back.