My son was also accelerated and was advised that sitting the Maths Ex 2 exam effectively negates the Adv Maths result. It can be a bit of a gamble if a student's Adv Maths mark is much higher than they are likely to score in MEx2, but then MEx1 also counts for two units rather than one, so these factors should be carefully weighed up beforehand. Unfortunately for your friend, it appears that s/he would have been on the all rounders list if they hadn't sat the MEx2 exam, but depending on MEx1 mark and with scaling, they may still have achieved a better ATAR with MEx2, than without doing it.
A similar consideration applies to deciding whether or not to resit a subject. Once you resit, the most recent mark is counted, negating previous marks.