The sword cuts both ways on this one...
On one hand, despite plausible or possible scenarios of user error, it would generally be considered good customer service to just replace the knife.
On the other hand, especially with such a public example, replacing the knife would tell everyone that it's ok to rip through drywall with Maxamet because if it breaks, no problem, Spyderco will just replace it. From there, more people who don't know better will start doing all kinds of hard use things with Maxamet and they'll expect a new knife when they break it.
I work in a business where the warranty on very expensive products is easily voided, and has extremely specific limitations. I deal with those customers face to face in situations very similar to this.
An easier solution is to monetize the problem. While Spyderco knives are not Legos, a blade replacement program could be made available for current models, maybe with some limited legacy support, at a price that makes sense, and in accordance with the level of steel. Example: Maxamet = $100 with everything else costing less.
If a Spyderco custom shop were to emerge in the future, a synergy could be leveraged through it's blade inventory.
Just because a knife has a Spyderco logo on it doesn't mean that its invincible. Taking the time to understand advanced steels before using advanced steels is highly recommended. However not everyone is fortunate enough to know what they dont know before purchasing such things. For that reason, it may be a best practice for Spyderco to include more details in the box, including warnings about usage restrictions, like they do for knives that lack corrosion resistance.
Ultimately the situation sucks for both the customer and Spyderco. Personally, I also would have sent the denial letter while fully expecting it to not be the end of the conversation.
I recommend calling Spyderco on Monday, be nice, and just go from there.