back-seat driver

Short definition from Wiktionary

Noun

1. 1
   An occupant of a large vehicle, such as a firetruck, who helps steer the vehicle from far in the back.
2. 2
   A person sitting (typically) in the back seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.
3. 3
   An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.