its also visible that he simply answered by inserting the text that the other user brought in - as if he was not plagiarizing, but the author of the answer
I don't quite understand this.
From what I gather, the situation is as follows:
The user found an answer to a question from another site. [perfectly fine]
The user decided to copy the exact text from the site. [perfectly fine*]
*(given negation of the following)The user did not credit the source of the original text. [not fine].
It's a little weird that you're being so vehement towards your friend/colleague's account and that they haven't written a single word of their own to defend themselves.
Forum rules are easy to find. I read them on my first day, IIRC.