If that is the only thing that's changed she's almost certainly going to find that the court won't grant the request. The kid should not be given the idea he'll get whatever he wants everytime he changes his mind. Frankly, the mother should not be enabling that mindset either.
I agree with you. However, if dad got custody because the child had the idea that the grass was greener at dad's house, and that was the primary reason why custody changed in the first place, then it honestly might be in the best interest of the child for there to be a reset. That is the only reason why I suggested a consult with a local family law attorney.