I agree with your sentiments t_richard on what appears to have happened here. Drugging anybody even an adult who doesn't know about it is contemptible.
However to offer up the evidence 10 days after the event, is very strange. If the mother was so outraged why did she not contact the authorities when she landed, if it was my child I would be concerned enough to do that in case the perpetrator did it to another child on the return flight.
To say that you will never look at a flight attendant in the same light again is perfect ********.
That would be like saying you would never look at any car driver in the same light again because you heard about one speeding once and causing an accident.