So it's this?
Where did it fail? At a guess I'd say just in front of the front pedals. .
From the OP's posts, I think you're right.
I wouldn't normally urge someone to sue, but this couple should.
The case will be decided by a county court judge who will apply common sense rather than engineering or cycling expertise.
The bike is a premium product, advertised as being able to carry a high payload - 260kg according to the website - so it must be sturdy.
It is unlikely two elderly people would be inclined to abuse it, or even be capable of doing so.
To suggest changing the rear saddle has anything to do with a catastrophic frame failure flies in the face of common sense.
The bike is only a few months old, so the couple are entitled to a proper repair, but more likely a refund.