EWL. I'll try to keep it brief & in point form.
1. You are not a trained mental health care worker.
2. You are not a trained mental health care worker.
3. Her own daughter, as lovely as she might be, was confident enough to be interstate so she couldn't have shown indications that would lead to close watching by professional carers.
4. You have already gone above & beyond, as opposed to others. (See my DJ post)
5. Uncommon sense has made...err..uncommon sense by saying that if it's on your mind so much, then you know you've done as much as you can to alleviate this problem.
6. It's time to hand this one to DFAT. She's paid enough taxes to deserve their involvement.
7. On what legal ground could you have refused to sell her a ticket?
8. You are not a trained mental health care worker.
Remember, there are sharks out there who would've sold her a first class ticket!