Maybe the first thing is to check how much current is being drawn while he car is off. Disconnect the negative terminal, put a DMM between the battery negative terminal and the battery negative cable. You should get around 0.3 amps initially, but after a few minutes (~10min) everything show go to sleep and the current should be below 0.05Amps, probably closer to 0.03amps. If it stays high, or is much more than 0.3amps, you have a short. First place to look is the alarm - most people remove it and throw it away. (If you don't know how to use a digital multimeter, youtube has plenty of videos on how to use the DMM to measure current. Harbor freight sells cheap DMM's for around $10.)
-Jerry