I think that you are correct, in that it is a chemical process where the recharger simple applies an electrical current to run the chemical process in reverse.

However, it really comes down to what the recharger is pulling over those 15 minutes. It actually may be drawing more current in fewer amount of time. So it may be greener, but not by much. So your dad could be correct also.

It would be interesting to test how much power it uses in 15 minutes vs a regular charger.