Hello,
Well, the usage of the plugin is not illegal, however, some content owners might be very restrictive for their content and might not allow it’s reproduction on other sites.
If he contacted you and asked you to take content down, you should comply and remove his content, because if not, he might create a DMCA takedown notice and fully take down your website.
However, after removing content you can contact this specific content owner and ask him if he allows partial recreation of posts (excerpts) with linking back to the original source.
Also, to prevent these situations from happening, you should check the ‘Terms of use’ page for source websites, there they should state if they allow partial or total reproduction of their content.
I hope this info helped.
Regards, Szabi – CodeRevolution.