In what situations and scenarios is it best to tell someone about how you feel?

Tweet

Last night, I had a long talk with my boarmate and discussed with her how I felt about a couple of things. The story is, she is younger than I am and also a batchmate of my sister in school. Lately, she leaves at night and comes back early in the morning. When I asked her about why she has been doing that, she replied that since we have a curfew in our boarding house and it's really late when she stays with her friends, she prefers to spend the night at her friends house and come home very early. I firmly told her that it is none of my business what she does, since I consider her as an adult, however, it bothers me in ways that she is not showing a good example to us and if she was my sister, I would definitely not allow her to be out with friends just for the purpose that it is 'FUN' to hang-out with them. She mentioned that her parents know that she stays with her friends, but I frankly told her that they don't know that she is mostly at her friends house. Simply saying that she is not telling the whole truth to them. I may be crossing the line and she may feel that it is not for me to worry, but if I ask you, do you think that it is really none of my business? That is one concern that I have about her, coming home early in the morning almost everyday - as per her reason, because it is fun to hang-out with them. She assures me that they are not doing anything immoral or illegal. She wasn't like this before according to her old friends, but I have no intentions of taking it against her if she changes, but it would be nice to know that the changes are for the better. How would you deal with it and what suggestions can you give?