I've done this for friends before. Just a few thoughts of mine on the subject based on personal experience:

-Say this first(or something similar) "I am going to be honest in my critique of your story. At times my critique may sound harsh, but I promise you I'm not doing it to be mean, so please do not take my feedback and criticism personally. I'm giving you an honest critique because I respect you, and I really want to help you with your story."(more often than not if you start by saying something like this, the OP will not get offended by anything you say afterward, so long as you don't say anything that will come off as unnecessarily mean.)

Then lead into this(or something to that effect in your own words):

"I think this is a good first draft for putting your ideas down on paper. I like the concept of your story and what you are trying to do with it, but right now it isn't working for me as the reader. Let me give you some details about that." (Lets you say something positive and lead into constructive criticism without having to drop the word "boring" or phrase "shitty first draft")

Proceed from there to give your objective and honest constructive criticism of the piece. Be honest with your friend, but don't be so honest that you come off as cruel; there is a fine line to walk between constructive criticism and cruelty. You can say what you need to say without making your friend feel terrible. And remember, even if you didn't like the first draft, you must give some positive and helpful feedback as well. Any time you give feedback on writing, no matter how bad the writing was, your goal should be to help the OP in some way. Don't just tell your friend all the things that were wrong with the draft, but then never give any examples of things or any suggestions on what changes to make for improvement on those things you don't like or found confusing. DO find a handful things you actually did like in the story, and tell your friend so they won't get rid of them!

Food for thought: Have you ever had a teacher in school or professor in college who was able to give you really good and helpful feedback for a shitty paper without ever making you feel bad? If yes, consider for a bit how exactly they were able to achieve that, and try to approach this the way they did. Best of luck.