To be fair to the parents, the adoption process nowadays is such a bureacratic shipwreck, it's an easy thing to overlook. We got our son a social security number and a passport within weeks of getting the final adoption decree from a court.
Does that mean that he's a US Citizen?
OK, I know most FReepers hate lawyers, but as a lapsed lawyer, I gotta say, cases like this are why you need one. A good lawyer, doing his job, would have advised the family during the adoption that they had to pursue citizenship.
Now, it could be that a lawyer was involved in the adoption and didn't know enough to advise properly. Then shame on them (probably too late for a malpractice case now). They shouldn't be doing international adoptions.
If the adoption was done by the parents by just filing papers (since he's marrying the mother, I can see where there might be some streamlined, do it yourself adoption process for these cases), these results point out why you should at least have a lawyer go over the papers or have a consultation so that you know you are considering everything that needs to be considered.
The kids sounds like a loser, but I can't imagine being sent, permanently, to some country where I didn't know anyone, etc. If they think he's a criminal now, just send him overseas and watch what happens to him....