As long as the crime committed is not severe, the military (the Marines in particular have a reputation for doing this) can overlook a prior record. It is also possible for a judge to sentence someone to joining the military in an attempt to straighten them out if their crimes are relatively petty, and any jail time served would have been fairly short. A kid in a street gang who as of yet had committed no major crimes but had an arrest for shoplifting or something, would be a good example. It is entirely possible that your ex is another-a 6 month sentence is right in that range where it makes it just possible that he was arrested, and given a deal to join the military.
From what I have been hearing about military recruitment numbers being down, I would guess that these kind of deals are being made more often now. Recruiters have quotas to meet, they are supposed to get a certain number of kids shipped off to bootcamp each month or quarter. So the harder it is to get their quota, the more likely people of less than sparkling character will be recruited. In any area where you don't meet the military standard, you are required to get a waiver. For instance there is a height minimum (I have forgotten what it is), if you don't meet it, you have to get a waiver-its just more paperwork the recruiter gives you, I myself had to get a waiver for previous drug use (Supposedly no military personnel are allowed to have ever smoked pot, but you can get the waiver when you join, and had I not been so determined to be honest, righteous, etc, I just could have lied since it had been over 30 days).
How old is he? One thing the military is very strict about is age-I think the cutoff is 31 or 32 for joining as a fresh recruit, you can only be older than that if you have had prior military service, then the number of years you served is deducted from your actual age. (34 - 4 years prior service = 30 years old for purpose of enlisting.)
There actually is good news here: IF he's telling the truth about joining the military, he will be forced to pay all the money he owes you. The military does not allow people to get away with not paying debts, when I was in I saw so many guys who had a checking account for the first time in their life who got wrote up or extra duty cause they bounced checks right and left.
John, ex US Marine
PS: Ask him what his MOS is going to be. 1) He should know what you mean, and 2) Marine MOS's are always 4 digit numbers, not numbers mixed with a letter like the army does. MOS stands for military occupational specialty; mine was 2141 (amphibious tracked vehicle technician).