How accurate is find my friends as to the devices exact location?

Pretty accurate, within say 10 to 30 feet normally.

The location accuracy can be gauged by the blue pulsing circle around their location pin. The wider the circle, the less accurate the location is.

Also note, that friends can choose what device the location is shared from, so the device location may not be their actual location. For instance they can choose to share location from their iPad, and leave that at home. So they can be 100 miles from home, and the iPad that was left at home, would still say the location is home.