As Andy says: if you upgrade the alternator to a higher output one ensure that you also upgrade the cable from the alternator to the common live on the starter motor as the normal alternators output is 65A NOT 100A ;)
Can i ask how you used the multi meter to find the 0.05A ???
Did you do it the same as i said???
When reading AMPS you have to measure as part of the circuit NOT across it :)
If he follows the instructions i posted he will find the circuit that is causing the 0.4amp drain, he can then go from there to find the problem ;)
You do need to be methodical in fault finding ;)