With the new details, I do agree that this was a 17-hour delay on a Type 3 flight with no extraordinary circumstance. Accordingly, the passenger is due EUR 600 from BA. Having been rerouted, BA had the obligation, at its option to refund his ticket, rebook then or at a future date. The fact that BA was willing to rebook on other carriers and cut the delay from 24 to 17 hours is a customer service gesture, but does not alleviate its obligation to pay the compensation. The only way that this would be the case is if BA offered a reroute which would have arrived at YYC with a <3-4 hour delay and had the passenger refused it.
I would recommend sending in a renewed claim which contains the information in not one word more than 2-3 short declarative sentences. Leave out all of the fluff. At least if the claim is denied again, there is something to work form.
The passenger has also not provided a location. But, if he has a UK address and BA again denies the claim, I would start the MCOL process with a Letter Before Action (form on the MCOL site). In that complaint it is even more critical to stick to what matters.