Cars running a CAN Gateway of Revision "L" or older, will potentially suffer from battery drain issues with RNS-510 units with a firmware of 1200 or newer.
Solution, if it's an A or B rev RNS, stick to a frmware 1200 or 1100 and you will have no issues. You do loose out on a little functionality and a tiny bit of speed that the later firmwares brought to the RNS, but to be honest, it's nothing to really worry over unless you want voice control etc. which can only be had with the later firmwares.
Option 2, upgrade the CAN gateway. It will be positioned either up above the clutch pedal (an absolute b*tch to get at) or behind the ashtray in the centre console. Be sure to get a genuine European version. The new CAN gateway will need to be recoded with VCDS (VAG COM) using the same tick list you took from the old unit. Basically, if you take away the time to actually reach the damn thing, it's supposed to be about 45 mins to swap and recode. The lastest revisions of the CAN Gateway are approximately "AD". The basic part number is 1K0 907 530. So if you currently have say a revision "E" unit, the part number will be 1K0 907 530E etc.
I have a Rev "E" unit in my car and couldn't be bothered in changing the gateway, so downgraded my firmware to 1100 and have never had any issues with battery drain. have left the car for well over a week as well.
If he wants to go ahead with upgrading the CAN gateway, quote the above part number to the stealer and he should be able to tell him the latest revision for his car. Will cost approximately £150 new from teh stealer.