Has he checked if neighbours have noticed similer? need to check with at least 2 each side as may be supply fault effecting only 1 phase, every 3rd house. If they have then report to DNO emergency fault number.
Is it over head supply? could be on pole or drop cable to house.
If none of above then inspect CU main switch for loose connections.
Take good look at cut-out and Meter connections, any signs of heat damage, "fishy" smells?
Sometimes just supply ups and downs, auto resets on supply can give short
break in supply almost instant back on.
Happy hunting