I'm going to stop covering for my co-worker regarding his work performance.
What's he doing?
His performance over the last few weeks has been really terrible.
Yeah, I agree with you. There's only so much help you can provide before constant mistakes point out his actual performance.
No shit, I'm sick of it and telling him over and over that his work is unacceptable.
At this point he shouldn't expect you to cover up his mistakes any longer.