Removing a playing partner's ball from the hole

There is 1 circumstance where this 'myth' does have validity.

The ball has to be 'holed' - defined as:

A ball is "holed" when it is at rest within the circumference of the hole and all of it is below the level of the lip of the hole.

17-4 is the relevant Rule (Ball Resting Against Flagstick) - and there's Decisions based on differing circumstances. 17-4/3 would probably the relevant one in this case (ball lodged against Flagstick removed by unauthorised person).

In Strokeplay, Should the ball not actually be holed before teeing off on the next hole (or all players leave green if last hole), player is DQ-ed from comp.

So, as usual, there is a sort of basis for the 'myth'!

Key first question though is 'was the ball actually holed?' If the answer was 'Yes', then just tell him and laugh at the pillock!
Interesting that he decided to come all the way to the green to tell you too!