It might be a good idea to call Dell tech support and they can determine if a service tech is required. That is a fair amount of play and the fact that she has no play on the exact same model would lead me to believe there may be a problem. You are under warranty and now is the time to call rather than some time down the road.
Thanks for posting and for sending your service tag information. I see a repair order has been requested, please post back to let us know if this resolves your issue.