From memory, the lower weight limit is around 55kg and the upper is 100kg. This is based on the lower and upper limits that the PC-9/A ejection has been tested at.
I am surprised, Blue Diamond, that your young friend was rejected. There was a girl on my pilots course who was under 55kg and a bloke who was over 100kg. Both ended up getting waivers.
Runaway Gun is quite correct when he talks about BMI. I know some RAAFies who are very fit from time in the gym, but they are classified as obese according to the BMI chart. Every year, before their annual fitness assessment, they have to get a certificate from the doc to prove they won't keel over on the 2.4 k.
Blue Diamond, your friend is on the right track with the appeal from the quacks. Best of luck.