You are not getting the point. If he wrote his piece completely without AI, why does he need to ask AI if his piece has AI in it? Do you wake up in the morning and ask a tool if a part of you is AI generated?
And lets say he does ask AI and AI says 60% is AI generated, what then does he do? Ask AI to humanise his own content.
As a web designer, I write content for some of my clients pages and I just don't bother with some AI content detectors (most especially copyleaks). And that is because I know they have been programmed to pick paraphrased content and also content produced by AI with prompts to write as humanly as possible. So, when you have originally human-written content, they tend to pick it up as AI.