This is not to be construed as legal advice, and I am not your attorney, A conflict check and engagement letter would necessarily be required before any retention or attorney/client privilege exists.
NO. That "someone" has a right to assert a registered trademark by notifying Facebook of the registration and your use of it, even if just a supplemental registration. It is unclear if "someone" and "another" are the same person. That's the problem with use of pronouns, as pronouns [especially "it" and "they", although here "another" and "someone" result in similar ambiguity] can lead to ambiguity. Registered patent attorneys are often trained to strictly avoid use of pronouns to avoid ambiguity.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
Can you sue someone for notifying the police about a crime? No. You cannot have a cause of action against a third party witness unless the testimony or accusation is blatantly false or malicious.