Accidental Picked up Ball in Medal Play

[Manx995]

A friend of mine playing in an autumn Saturday bottle competition thought it was a stableford competition. He got into a little trouble on the first hole so thought as it's stableford he would pick up and move on. Only when he had put the ball in his pocket did his playing partners say it's a medal competition. So was he right to disqualify himself or could he have replaced the ball as it was a genuine and innocent mistake? 

[DaveP043]

23 minutes ago, Manx995 said:

A friend of mine playing in an autumn Saturday bottle competition thought it was a stableford competition. He got into a little trouble on the first hole so thought as it's stableford he would pick up and move on. Only when he had put the ball in his pocket did his playing partners say it's a medal competition. So was he right to disqualify himself or could he have replaced the ball as it was a genuine and innocent mistake? 

Per Rule 9.4 regarding a Ball moved by the Player, the Player incurs a one-stroke penalty, and must replace the ball at its original location.