It can happen especially with Pakistani Qualiy Control
If the car drives right and he can't notice any other problem, most probably there ain't any.
Not much damage could have happened in 700 KM, u didn't mention the amount of gear oil leaked and how much was remaining.
we mostly use our cars for short trips so I don't think much damage could have happened, I recently read on quora a person drove 2 states without a drop of gear oil before his gearbox gave up. If there was any damage occurring, you cousin should must have felt it in way of hard shifting or by excessive noise. (the person I'm talking about also kept ignoring the noise during his drive which was present before he set off to cross two states)