Back to the thread (interesting as the other discussions are), I can confirm that this occurred on flight AA709 on 13 July. My friend agrees that the bank excursions may not have been to 90 deg but that they were excessive compared to what he had ever seen before in a commercial aircraft. He also agrees that the aircraft may not have been at 20,000 ft as it appears that the aircraft was probably turning on to the approach when this happened. However, he and his companions were definitely concerned that the aircraft may crash!
I suspect that the incident was due to some aggressive handling for whatever reason, possibly due to atmospheric disturbance or an ATC input. However, the debate that has ensued has raised some lessons that everyone in the industry might like to consider.
1. An eyewitness's perception may be markedly different from reality due to the nature of visual and acceleration cues that they have received. They are just reporting an event as they perceived it. If they have been unsettled by an occurrence they need to be convinced of the reality so that their mental model can be refined. They then may have a more accurate perception if such an event occurs again.
2. Good use of the PA after an unusual event can allay many fears and uncertainties and restore passenger confidence. There are both humanitarian and commercial reasons to do this.
3. Scepticism of reported incidents such as this is healthy, cynicism is not so useful!
4. Thanks to Wino's reply, my friend is much happier as some of the uncertainties and questions that he had have been answered.
If anyone has any more details of this flight, I would still be interested to hear them.