backseat driver

/B AE0 K S IY1 T D R AY1 V ER0/
noun
  1. 1

    A person sitting (typically) in the back seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.

  2. 2

    An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.

Source: https://en.wiktionary.org/wiki/back-seat%20driver, https://en.wiktionary.org/wiki/backseat%20driver