The issue here is that his current citizenship does not change that fact that when he first entered into the military, he committed a crime. I wish I could tell you differently here, but the law is very clear on the subject. His initial enlistment was fraudulent and the military tends to be upset when that happens.
Now, his command is not legally required to court martial him or to process him for separation, but that is a command choice. He will not have a legal right to not be processed. If command chooses to let it go, which is highly unlikely but technically possible, then he'll be able to continue to serve.
The more likely result is that a commander simply separates him from the military, but he doesn't get a very bad discharge....just a general discharge, based on his prior service. A commander, respecting his prior service, can choose to NOT charge him criminally, which is a good result here.
I know that you'd like me to say that he can leverage his military career and deployments as a way to get them to let him continue to serve. I wish I could tell you that, but legally that is a very unlikely result. Honestly, the best that he can hope for is avoiding court martial for having engaged in defrauding the military for 10 years.
I'm sorry. I'm just trying to be honest with you here about the probable results.