If we're talking 5e, do the rules say they can be used on PCs?
The PHB states that players decide what their characters think and do. (Somebody else will have to pull up the exact quote; I gave my PHB away.)
There is a general rule of "specific overrides general".
Everything on attribute rolls and proficiency bonuses ("skills") is described from the perspective of PCs.
So there is no specific rules that NPCs rolling persuasion, etc. overrides the general rule that players make their own decisions.
We had a big debate on this a few years ago and the best/only evidence that NPCs get to "use social skills" (cited, ironically, by the person with whom you are arguing, who I have blocked) was that in the entire Monster Manual there can be found a handful of entries with bonuses to social skills. But there is no description of how they are supposed to be used, which could be on other NPCs, or if they are just there for colorl