They allow to go into the military instead of convicting you of a felony. Once you are convicted you have to have it expunged of your record. The military does not take convicted felons.

Your answer would, more properly have been asked in military, but here is something for your friend to consider. When his probation is over, he may be eligible for a waiver. However, if he approaches the judge and asks him to drop the probation because he is trying to enlist and the judge does so and enters it into the official court record, your friend will be PERMANENTLY disqualified from enlistment with no possibility of a waiver.

the military is full of feckless, irresponsible piss heads so it is only right and proper that your friend should be allowed to join the other cowboys who think it’s fun to go shooting foreigners!

Would be best to talk to his probation officer or overseer about this. They usually have the say and the ability to do something like this, and work with you if you can show that you are actually going to join. They might ask him to at least finish 6 months of the probation or maybe nine which would be half of it before considering it though.