So... He already knows that his ability doesn't just copy skills, but also sometimes improves those of the original holders (chapter 17)? That's a risk that they can take with this snake?
And... The girl with mind control powers just leaves? They just let her go? She is never going to be mentioned again, is she?
Also note that they didn't give the protag the "bad guy" power that the original holder had.
Edit: and it doesn't say why the skill didn't work on him, did it? Like, he didn't actually do anything to avoid being affected.