To start with the answer, this is not Outside Assistance. Even though it may look like Outside Assistance, and spectators should not intervene in a match (unless they want to stop them to call a judge), we don’t issue a Match Loss here. The definition of Outside Assistance specifically says: “play advice”, and providing comprehensive rules information is not play advice. Otherwise, judges would be giving play advice all the time, but we don’t.

You should talk to the spectator and explain why his behavior is not appropriate, with an instruction not to repeat it. While this particular behavior doesn’t have an infraction associated with it, it is still prohibited by the Magic Tournament Rules, which require spectators to be silent observers in a match.

Sometimes players won’t realize the difference between pointing out rules interactions vs. a strategic mistake, and could get themselves in trouble in a different circumstance. Pointing out missed triggers is also considered to be Outside Assistance because of their nature (opponents don’t have to point them out, and remembering one’s own triggers is considered a skill).

Explain to spectators that a match is between two players, and if they see a mistake, they should call a judge (and may ask the players to pause at Competitive REL). Instruct the players to continue playing with the token on the battlefield, and don’t issue the players any infractions because no Game Play Error has been committed.