We can't say that it was a mistake because it is not even our Bitcoin and it's not our business to decide what to do with the Bitcoin of other people.
It might be a waste of opportunity to others, but maybe that someone has a valid reason why he did spend his Bitcoin to pay a house rent, (1) For experience, (2) maybe he just don't have any money on that period of time. For me, as long as you spend it on the things you need, it will never be a waste.