Always speak to someone privately when giving mussar, and do it lovingly with friendship.
NEVER embarrass someone in public, except to save a life, though I cannot envision such a scenario at the moment.
At one time, I had teachers who would ridicule some students and that was just awful for the students who were humiliated in class by them. Thankfully, I was not one of them, but in retrospect, I can see how it had to have been very hurtful to them.