I would say that with a shortfall of 2 marks, provided the friend can present strong evidence, an Appeal is worth it.
Initially your friend's son must present the same information as for all Appeals - the best possible proof via academic evidence that the child is suitable for a Grammar School, regardless of what occurred on the day. There is advice on this throughout the Appeals section and on Etienne's "Appeals Q & A" here
Your friend will also need to present written confirmation from the school of what happened on the day.
The panel are likely to ask questions such as: How far in to the test did he leave the room? How many questions does the child think remained unanswered at that point? Was the child showing symptoms of illness on the morning of the test before leaving home? Was the child sent home after leaving the test? Has the child previously suffered from headaches?
The panel will be trying to weigh up the academic evidence against the shortfall of two marks, and then trying to decide if leaving the test early was the key influence on the child's score. In this situation high CAT scores (or comparable test results) would be very helpful, as they would show the panel what the child is capable of achieiving in tests under different circumstances.
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