First of all, hire an Enrolled Agent who has experience with GA taxes.
You should be able to get copies of his military pay records from the branch of service he was in, also should be able to get proof he WAS in the service during that period.
I would also ask for proof from the state about the income he supposedly had in that state.
Just for your reference, Married couples where one is military are often permitted to file in different states as though they are single or as MFS for state purposes if the military person is not a legal resident of that state, and the spouse worked there.
Don't try to handle this on your own since you don't have the paperwork they are asking for. What they really need is a pay stub from that time period, the W-2 won't show them anything; it does not list state of residency.
This post does not create an agreement to represent you before the IRS, nor does it invoke confidentiality regulations. Postings are based only on the information provided and you should consult a tax professional in your area before relying on information contained in this post.