The odds are that a fault in the charging system wouldn't likely drain the battery, but just keep it from being recharged.
I suspect he has a major drain, more than my suggestion of radio memory drain. With one of my older Alfas, I put a load meter on the battery to read the drain, and then pulled one fuse at a time to see what happened, what the meter read. It's how I found a similar fast drain. Told me which circuit had a fault.
This, provided that the battery itself has been found to be ok, ie, no bad cells, etc.
1989 Milano, Shankle Sport
1991 164S, stock
1994 164LS (~Q)
1972 Morgan 27
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6