You can't allow a take-back in rated games. However, what you can do is purposely not take advantage of the mistake or blunder. For example, if your friends daughter has left the Queen hanging, then don't take it and allow your friend the chance to get it out of harm's way.
Do random move, he moves back the piece to the same place and then you do the same then you are in the same position. you will have to trust your enemy.