General:
* Observer
* Spectator
* Witness
* Looker-on
* Onlooker
* Passive observer
* Non-participant
More specific:
* Neutral observer: If the bystander is not taking sides or actively involved.
* Innocent bystander: If the bystander is uninvolved in the event and is potentially at risk.
* Casual observer: If the bystander is not paying close attention.
* Silent witness: If the bystander is observing but not speaking up.
* Apathetic bystander: If the bystander is showing a lack of concern.
The best synonym depends on the specific situation and the tone you want to convey.