How to Replicate:
Set "Friendly Fire" to On
Add a friend
Try to hit it, the hit will work
Set "Friendly Fire" to Off
You can't hit your friend
It is Inverted. Friendly Fire active means no PVP, right?
MCPE-172008
Friendly Fire only has effect if set to False
Resolved
Invalid
0
The phrase "friendly fire" refers to hitting your friend. So when it is set to on (or /gamerule friendlyfire true) you can hit other other players, when it is set to off (or /gamerule friendlyfire false) you cannot hit other players.
See https://idioms.thefreedictionary.com/friendly+fire for help understanding the phrase.