Word-for-word copying of relatively small amounts is only allowed as evidence / supplementary material, and even then must be properly quoted and attributed.
A quote should never be a full post, attributed or not, and presenting it as one's own is strictly forbidden.
The best course of action when you encounter something like this is to flag one of the user's plagiarized posts with a custom flag and tell the moderators you found plagiarism, with a link to a word-for-word source of the actual text.
If it's a good question worth keeping around, you could rewrite it in your own words and perhaps a link to the original question. If it's not, it will probably just be deleted.