cliché: a back seat driver

Meaning: one who criticizes from the sidelines (definition).

Example: From the other side of the counter, Marge made continual suggestions on how to mix the recipe, in her typical role as back-seat driver.

Origins: From the modern day automobile passenger who freely comments on the driving habits of drivers. (Source.)

Rewrites

• back seat adviser
• rumble seat supervisor
• rear seat moderator
• coach seat pilot
• over-the-shoulder editor
• offsite oversight
• blind guide 

Discussion: Not sure these are all equivalent rewrites, but they all give a flavor of the metaphor intended.