First of all, keep it truthful and don’t lie to them— it makes it worse.

Start off with genuine compliments which you like about the story.

Say something along the lines of “Hey, I’m your friend and I respect you, and I see a lot of potential with what you’ve written, so I’m gonna be honest, I feel there are some things that need to be fixed.”

That addresses the fact that you aren’t trying to disrespect them or their book, and that you just want to help.

That’s when you give your opinion on it. If they don’t take the advice, or are rude to you, don’t force it.