If I follow, wouldn't leaving his voltmeter in series and pulling fuses eventually get him there?
That would tell you something about the battery's state of charge, but it wouldn't tell you exactly which circuit is discharging the battery.
Of course some will want to argue about leakage currents and such.
But as long as you are going to go to trouble of creating a series connection, you might as well flip your multimeter to "A" and read amps instead of volts.
Just make sure you have put the probes into the "current" connections. Also make sure you've removed all rings and watches before having anything todo with an automotive battery.