Hi, It is recommended that new pads are fitted, your garage is correct as fitting worn pads to new Discs can cause adverse wear, the first Garage should of emphasised this.
If I were you I'd probably get the Discs looked at to make sure they are not too low, if still safe just wait until the Pads need replacing next time and replace the Discs then.
No doubt people will tut and comment on this bit but... I have fitted worn Pads to new Discs and had no ill effects.
2010 Titanium 2.0 TDCI Panther Black.