Q. 친구가 실수하면 놔둬야할까요?
There are lots of old saying about the importance of using one's tongue. Those usually recommend people to hold their tongue speaking about the words related to mistakes or bad things that others had made. However, I would speak of and suggest my friends faults rather than not telling it.
Mentioning about your friend's fault to that person, who is especially close and will be the one for lifetime, is not optional but essential thing to do as a best friend. If he or she is very meaningful to you, you would also want your friend to become a better person than before. To make your wish come true, you should help your friend to realize him or herself about the shortage of him or hers. Then, it will be possible for your friend to revise it. In my personal experience, even though the moment of speaking is difficult, they all appreciated in the end.
Another reason why I prefer the latter is, the mistakes your friend made is related to you and your friend, the emotion between of you will get worse. If your friend made small mistake, it would be nothing to you. But, those small things keep compiled together, the moment you complain and burst out about those things would evidently come someday. It would be much wiser to solve the problems earlier than the big storm come, as the relationship could be broken after the huge conflict. In my case, I try to tell my friend's mistake as soon as possible for I had experience of loosing one before.
For the reasons that I mentioned above, I strongly insist to tell my friend's mistake at the very moment so that there are no chance for the odd emotion between me and my friend would never exist and also for my friend's development. As the result of this, I still have a friend whom I can call as a friend for a lifetime.