I'm going to tell my best friend that he will most definitely fail a course
Why would you tell him that, what makes you think your friend will fail?
There are many reasons, but the most important of them is that he doesn't study
You should let your friend handle his own education. If you're concerned, maybe you can help him develop better study habits. The two of you can spend time together while studying.
It's too late, my friend is an idiot. I'll see him fail.
You shouldn't just write your friend's abilities off and assume he won't be able to do well. If you convince him to study, he might be able to get a good grade!