I can't speak to RI.

IF the cancer is subsequently deemed to be job related some/all of the pension can be deemed disability & therefore not taxable at the Federal level.  That determination can be done retroactively, and a correct Form 1099-R issued.  But that will not happen by April 18, 2023 (or maybe not by 4.15.24 even...)

I would file an extension to see what/how fast things may happen.  Or if there's a refund, file & amend later.  

But *YOU* or your client can't just arbitrarily determine that some of the pension isn't taxable. 

There's no blanket rule that just because a firefighter gets cancer after retirement that some of the pension isn't taxable.