Tell him to try turning off the sensors first, 2 little switches inside the car that are located in the drivers side door pocket towards the rear of it, pull both switches towards you so they light up red, then set the alarm, see if it still goes off.
if it does, the alarm is located behind the felt panel in the boot, but it has its own battery so someone else maybe able to help you pull a fuse etc?
Like its been said, these sirens are *****, I thought mine was bust when it went off but the guy told me thats your alarm.
In fact my alarm was going off on my front few weeks ago in that mega heat whilst I was in my living room and it was the neighbour that knocked my door to tell me its been going off for atleast 3 mins!!!!...... So what do neighbours have that their next door neighbour doesnt..