You know, I am having an irritating week and I am irritable, and I am slamming onto you unnecessarily. You are right and I humbly apologize to you.
The simplest way to look at this is to establish whether handloads actually nullify warranty coverage, in the case of other companies, it isn't in question.
If the loads didn't void the warranty, the point is irrelevant. He is fully entitled, and in a certain degree,obliged to send it to the maker for examination.
Whatever was in the load, the unsupported chamber probably allowed the case to fail, when it might not have. Unsupported chambers are a potential problem that the maker put in on their own. Any weak case, even factory presents a risk with them, and this risk may be lower in a fully supported chamber.
(Again, this also begs the question, is the case actually unsupported, or does it actually have steel support all the way to the web of the case, with none of the thinner body exposed? Unless thin body brass is exposed, we again slide into another issue.)
The final question is, should that handgun have gone through catastrophe failure, essentially because he used unapproved, yet still (probably) safe loads, that may have failed, on l y because of a design that isn't as strong as others?
Sending it back is his right. When there, the company will probably find no facts that conclusively prove that shooter error, and being smart people, they will recognize that there are no contractual or legal grounds to deny his claim. They will almost surely make some sort of concession, unless he very clearly violated warranty in some grleviou way, like performing modifications to parts subject to mechanical stress.