I would have taken other passengers' names as witnesses then pushed the issue. An arrest would require a local cop, which might have a different opinion, plus a jury would likely take a dim view of the situation. (In a subsequent civil suit as well.) No guns, but I have both lawyers and money.

Lastly, the TSA is not allowed to confiscate most items that don't violate a law, only to prohibit flight with them. Your friends could have walked back out of the checkpoint and mailed their phones to themselves.

Anyway, with any luck, the TSA thieves who did your friend will f@@k the wrong guy and end up being the "victim of a botched mugging" or wearing concrete cowboy boots in the East River a year or two later. Karma's a *****.