Yes he might well have issues if his car is an earlier model (and still quite possible on later ones up to 2008). The battery gets drained via the CANBUS Gateway which connects to the headunit (this gives it all the info like when the lights are on to dim the HU lights, when you speed up to increase the volume etc etc).
The solution is to replace the CANBUS Gateway with a newer version and get a firmware update (a bit of internet hunting came back with CANBUS Gateway version B and firmware version 1200). The dealers should really know this (but am not surprised they dont).
I would be asking for my money back on the new battery if he paid for it as there was no need for one.
HTH.