I work in the tech department for Bradford and a caller brought this link to my attention.
While the installation manual mentions water softeners and the need to replace/serivce anodes on a more regular basis, it does NOT state that use of a water softener will void a warranty. Removal or cutting off the anode, that WILL void the warranty.
I would be more than happy to talk with you or that plumber to better understand the background behind the denial.
Our tech group is open 24/7, located in Middleville Michigan. Give us a call if you have questions.