The goal with constructive criticism is to bring something to someone’s attention to assist him or her in doing better, improving themselves or the situation. Unfortunately, constructive criticism is usually met with defense.
You can start with the Compliment Sandwich; offering a compliment, then the constructive criticism, followed by a compliment. This helps to keep the situation positive. Yes, it may be “sugar coating”, but if it helps the “medicine” to go down, then it does the job.
Make sure to focus on the point, not the person. It’s the action that you are applying the constructive criticism towards, not the individual.
If the situation allows, add a bit of humor. By approaching the topic in a light-hearted manner, it usually helps the recipient to be more open to the subject.
And of course how the person responds to the offer of constructive criticism is completely outside of our control. Keeping in mind, we can’t control what other people do or say, the only thing we can control is how we react. So know that you are doing the right thing by offering the advice, and hope your friend receives it in the same spirit.
For more on dealing with how others react, here is a previous post on “Hurt Feelings”.