Players can exploit an infinite amount of emeralds with cured villagers. Are there any plugin available to prevent this?
Yes, from the 1.15 patch notes: "Librarians now sell one bookshelf for nine emeralds, instead of three bookshelves for six emeralds." If it's that you're talking about.
God everyone I see who is still on 1.14 kills me inside. PLEASE update. It's already been proven and widely discussed 1.15.2 is sooo much better, as they focused to fix memory leaks and other performance issues. You shouldn't really have any conflicts, or perhaps even have to, updating any plugins.
Unless he's using plugins that aren't updated to 1.15 yet? It's not always that easy to "just update" *Cough* *cough* NMS
