I'm going to delete my friends plagiarized content that he stole from me
Before deleting it, did you let him know that you knew about it?
Of course, I review all his works for him and that involves checking for plagiarism.
Plagiarism is wrong. It sounds like you did everything you could by letting him know, so go ahead and delete the content. Your friend will not be happy but in the end will learn a valuable lesson.
I already did. I never even knew there are people who actually do these things.
You did the right thing here, both in terms of basic honesty and to hopefully stop your friend from getting into real trouble.  I hope that you can still talk to your friend and let them know why this was wrong, and why you had to delete that content.