This sounds like a terrible experience and I hope that the passenger was able to enjoy a business class seat in the end.

If you do go down this route you will need to be sure to stick to the facts, avoid hyperbole and not jump to conclusions! A few of your posts above may be guilty of not adhering to these points, so please be careful!

The most important thing I would consider is whether or not QR offered rerouting in the ticketed cabin or re-accommodation in J on another service the next day.

If they did offer rerouting or re-accommodation then I would consider they have made reasonable efforts to fulfill their obligations. If they did not offer rerouting or re-accommodation then you may well have a strong case against them.