Reciprocity applies to a CCW license issued by another state. If your friend is considered an Illinois resident and has a valid FOID, he could go through the same process as someone who has lived in Illinois all their life.
Prerequisites for a non-resident permit are
NON-RESIDENTS
•I am at least 21 years old.
•I have not been convicted of any Felony.
•I have not been adjudicated as a mental defective by a court of law.
•I have not been involuntarily committed to a facility for the purpose of mental health treatment.
•I have not been admitted as a voluntary patient in a facility for the purpose of mental health treatment within the past 5 years.
•I have not been addicted to narcotics.
•I am not intellectually disabled or developmentally disabled.
•I am not subject to any active Order of Protection.
•I have not within the past 5 years been convicted of battery, assault, aggravated assault, violation of an order of protection, or a substantially similar offense in which a firearm was used or possessed.
•I have not been convicted of a misdemeanor crime of domestic battery or substantially similar offense.
•I have not been adjudicated a delinquent minor for the commission of an offense that if committed by an adult would be a felony.
•I have not been convicted or found guilty of a misdemeanor involving the threat of physical force or violence to any person within the past 5 years.
•I do not have 2 or more violations related to driving while under the influence of alcohol, other drugs, intoxicating compounds within the past 5 years.
•I am not subject to a pending arrest warrant, prosecution or proceeding for an offense or action that could lead to disqualification to own or possess a firearm.
•I have not been in a residential or court-ordered treatment for alcoholism, alcohol detoxification, or drug treatment within the past 5 years.
•You may be eligible if you satisfy the above and no objections are filed by law enforcement. (Section 15)
At any rate he could apply for a FOID with a copy of military orders documenting assignment to his current post.
Mark Owensby at Rampart Range is a top shelf instructor.
http://www.rampartrangeinc.com/index.html