Was charging with 3rd party accessory fast before? If it was then that's odd - if it was always slow - perhaps try a more powerful apple charger? (e.g. 10W one?)
How long have they had the iPad 3? It might be that the battery isn't in as good condition.
iPad 3 is quite power hungry due to the retina screen and older chipset; I had to get the 10W wall charger just to do it faster.