Yeah, it happens.
Imagine what she could've achieved if she had been consistent!
The smartest student I have ever taught only got an ATAR of 95.2 I think, but he never studied. He was smarter than the students I taught this year who got 99+ ATAR's and smarter than me when I was in Year 12.
Some people just naturally have it, some need to work harder than others. Just remember, although it may seem like he "wasted" his talent as he is capable of 99+ ATAR, HSC means jack all.. He will be able to do whatever he wants in life.
Smartest guy I have ever met only did enough to pass subjects at university, and I mean get exactly 50. He knew the answers to everything, but he would just allocate which marks he needed to pass and just do answers to them.
2nd Year university and he was already working part-time for some huge engineering firm on an insane amount of $$$