Was the Wrongdoer’s Baptism Valid?
46. When dealing with wrongdoers,the elders should not raise questions about the validity of the individual’s baptism.If the individual raises the issue, the elders may refer him to the February 15, 2010,Watchtower,page 22. Further information on rebaptism can be found in the February 15,1964,Watchtower,pages 123-126, and the March 1,1960,Watchtower,pages 159-160. Rebaptism is a personal matter.
48. There are rare occasions when it is obvious that the baptism was invalid because serious wrongdoing did not cease before baptism, even for a brief period of time. For example, it may be that at the time of baptism, the individual was living immorally with a member of the opposite or the same sex, was a member of a non-neutral organization, or some-thing similar. If there are questions, the branch office should be consulted.
Source: Shepherd the Flock of God (secret elders manual).
According to these rules, your buddy may get what he wishes since the military is a 'non-neutral organization'. But I doubt it will really fly.
Especially if he ever signed some paperwork (for Bethel service, RBC or whatever) after he left the military reserves; then they might argue he signed with Watchtower showed his decision to be a true Christian (e.g. a JW).
And fear not, even if his baptism is declared null and void, the effect will most likely be the same as when DFed or DAed: the big shun will start. After all, those who ever chose Watchtower Jehovah cannot simply walk away.