“Leave it, “mine”

[one]

In law you're supposed to let them complete the penalty and then allow a retake if they miss.

It depends if the offence happens before the referee signals for the kick to be taken or after.

There is a bit of redundancy in the way the laws are worded though. In some cases (say a faint at the end of the run up) there is no point for the kick to be taken, the result is IFK to the defending team regardless of what happens if the kick is taken.