He is currently a naturalized American citizen. For him to lose that status, he would have to be stripped of it by a legal process, commonly called denaturalization. [0] It is not enough for people to accuse you of having committed fraud in naturalization.

There’s no valid path to naturalization

I think it is correct to say that your naturalized status can be unwound if a prosecution against you is successful, but until that happens, you are considered an American citizen. Presumption of innocence and all that American stuff.

[0] https://en.wikipedia.org/wiki/Denaturalization