Below are a few guidelines for tactfully confronting someone about their behavior:
Do No Harm: Try to gauge whether or not the person is ready to hear it before you blurt out any truth. Ask yourself what you hope to accomplish by getting involved.
Be Sensitive, Not Superior: Choose the time and place wisely, making sure you don't set up the other person for embarrassment. Give compliments in between your comments.
Keep Your Emotions in Check: Think twice before speaking once or popping off in anger.
Depersonalize the Issue: Describe a parallel situation as an example to mirror for the person. Express your concerns using "I" statements rather than accusatory "you" statements.
Be Very Brief: Get right to the point, and then say no more. Give the person some time to recover and then respond.
If you make a genuine attempt to use these guidelines, the person you are confronting will feel less defensive, and remember, the less defensive you make the person feel, the more likely you are to get results!