In short, employers do not have to report on Form W-2 or Form 1099-MISC any
payments made or benefits provided to volunteer firefighters that are excluded from
income under section 139B. However, employers must report on Form W-2 or
Form1099-MISC amounts that are not excludable under section 139B, or any other
Code provision, depending on the status of the volunteer firefighter as an employee or
independent contractor.

https://www.irs.gov/pub/irs-wd/10-0079.pdf

--So it should have already been excluded on the volunteer firefighter's W-2 or 1099.  (The exclusion is a maximum of $50 a month.)