You can use a rel Canonical to avoid Duplicate content errors on Google.
For each article on your client's web site add a meta in head with the url of the origin article.
<link rel="canonical" href="https://original-website.com/article-url" />
For the official source, you can check this page :
https://moz.com/learn/seo/duplicate-content
About your update, this definition given by Google makes clear if you copy 1000 words of 3000 without rewriting it, this will be Duplicate content.
Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar.
https://support.google.com/webmasters/answer/66359?hl=en
The only way you have not to make DC is to rewrite those articles or add a Canonical. So, Yes it's still ok to add Canonical tag ;)
An easier way could be to add a no-index meta tag on your pages this way you are sure your copyed articles don't harm original ones SEO.