It's okay to go get a judge, if they see something wrong.
As a judging technique, the spectator should be reminded to let the players enjoy the game. Anyone can feel stress from playing the game, and miss something. Unfortunately, by having a spectator jump in, there can be even more stress added for the player.
Generally speaking, if someone is interfering with a match and has some emotional interest in the game, I've asked spectators to leave the table area...it's for their sake (I don't have to get tough) and for the players (they know they won't be distracted that way again).