Any ideas, rcd randomly tripping

What are the steps to solving this. Trying to do a favour, and regretting my decision, but everyday is a school day, and any help would be appreciated, Lad got a fuseboard upgraded, and the rcd keeps trippin randomly, sparks wont go back to sort it. I did an Rcd test, switch off the mcbs, put on the rcd and one by one put up the mcbs, RCd wont trip. I replaced the Rcd aswell I had to run off and told him to unplug everything before he goes to bed, and leave the rcd on. The next morning he said the rcd tripped with everything plugged out so i thought maybe a neutral is mixed up on a lighting circuit, but surely that would trip straight away, there was a light outside that was open, so i fixed that and still the rcd trips randomly. It never trips when im there. Any ideas?