How long have you need here, months? 

Many thieves give themselves away after the crime has happened.  In fact, that's how the most serious cases (murder, rape, etc) get themselves caught.   The criminal brags about it, or they make conflicting statements, or they say something that can easily be proven untrue.  It's harder to detect in written form since the author has time to phrase his words and can look up past details, but if you are stupid enough and greedy enough you will eventually fuck yourself.

In this case, I think he's just a troll and FMC has the right idea - we should ignore these anonymous bozos by default, as they cannot hurt anyone.   Anxiety and stress come from judging other people who have entirely different morals than you do.    How much would you care if a person who believed the earth was flat called you an idiot?   Prob not much.