i think you should correct this early. the earlier you correct it the easier it will be for that student to drop their bad habit.
that being said, make sure correction comes at the appropriate time. if it's just free talking time where the goal is just to get students comfortable speaking, let it go. but when it's time to focus on pronunciation, drill them over and over until they get it right.
Here's a
game to use to make pronunciation practice fun.
eric