IANAL, but I think this would be a tenuous claim. If I see you with a camera and do something while you are pointing the camera at me -- with the express stated intent of having you film me doing it, no less -- I don't think I would get to (credibly) claim "hey, not my responsibility, I just happened to be doing that thing and you happened to catch me doing it on camera."
The cop has no clue whether or not the person is actually filming or not, no clue what the person intends to do with the video or audio, and most importantly - has no involvement or control in either the recording, editing, licensing or distribution of the material.
There's no way in high heaven that someone's going to be construed to be in violation of the 'distribution' artifact of copyright law if they have no participation or really even awareness of the distribution itself.
This thread is full of absurd arguments.
The normal way you play music is for your own enjoyment. It's incidental if other people overhear. He said that wasn't what he did.
The harmed party needs to be the owner of the copyright, because people listen to this instead of paying them.
Unauthorized public performance is a violation of the copyright holder's exclusive rights. Market effect is only 1 of several factors examined together for a fair use defense.
They have no reason to sue this guy that doesn't apply to millions of other cases that happen every day.
That said, he should be fired for reasons that have nothing to do with copyright. It is an obnoxious attempt to avoid accountability.