If he's got a decent mega tester there shouldn't be a problem if he uses the 250volt testing scale to test any circuits with PC's or other IT equipment.
The eliminating game would be a possibility if it was just 1 or 2 circuits causing the tripping. To play the eliminating game though you need to disconnect circuits on both the live and the neutral side, at the moment you're only disconnecting the lives when you use the breakers (MCB's) so that's where a lot of your confusing results will be coming from. RCD's (earth leakage breakers) are monitoring the live and neutral simultaneously so they can still trip to a fault on a circuit even if it's no longer live after you've knocked down the single pole breaker and this was kinda where my joke about the binary message was coming from.
The other way of monitoring actual leakage current is to use a suitably sensitive clamp meter around the live and neutral together. The current it will display is the difference between the current through the live and the current through the neutral which is basically the current leaking to earth.
The earth wires all stay connected to the main earth bar no matter how many RCD's are installed. It's the neutrals that need to be identified and split up accordingly. Installing a second RCD isn't always plausible due to space constraints. You need enough room for the RCD itself plus enough room for an extra neutral bar.
Installing a second RCD can help if your problem is due to the cumulative effect of many small leakage faults but it can also actually highlight faults that were previously not causing problems. You need to check each circuit for borrowed or crossed neutrals otherwise it can actually cause extra complications.
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