You need to make sure that the filter does not lose an ammonia source of some description, and does not dry out. So, once your uncle's last fish leaves the tank, you need to keep feeding the bacteria with ammonia, until you get hold of the tank.
If you do this, the filter will have remained cycled, and you can fill the tank with fish straight away, but test regularly to make sure there are no unexpected spikes.
If the bacteria are starved of either ammonia or oxygenated water, they will start to die - the amount you have left will depend on how long they go without ammonia or oxygenated water. Then when you start to re-establish the cycle, the more bacteria you have left, the shorter the cycling period.
