1) Bystander 2) Eyer 3) Eyewitness 4) Looker-on 5) Observer 6) Spectator 7) Viewer 8) Watcher 1) Bystander 2) Eyewitness 3) Looker 4) Looker-on 5) Non-participant 6) One on the sidelines 7) Passer-by 8) Passive spectator 9) Rubberneck 10) Someone who looks on 11) Spectator 12) Spectator, witness 13) Viewer 14) Watcher 15) Witness 
[n] - someone who looks on
looker-on noun someone who looks on