Setting the scene:
A player tees off from the senior men's teebox which is adjacent to the ladies tee box. She realises her error when her fellow competitiors tee up from the correct tee box. A fellow competitor says that there is no problem as she "has not gained any significant advantage" The player does not correct her error and completes her 18 holes.
This is totally incorrect.
The player, under Rule 11-5, is required to correct her error by teeing off from the correct tee box and incurs a two shot penalty. This must be done before she tees off from the next teeing ground. If she does not do this, she is disqualified. The shots played from the wrong tee box do not count.
If the men's senior tee markers had been directly behind the ladies tee markers and within 2 club lengths of the ladies tee markers, there would have been no penalty. Definition of the teeing ground is between the designated tee markers and then 2 club lengths in depth.