The release had a bit of information I hadn't seen before, and which explains the basis for the charge that Nachman had misused his Diplomatic passport.
Nachman also pleaded guilty to misusing his U.S. diplomatic passport by using it for personal travel to Brazil in November 2007 after he had been removed from that country by the U.S. Ambassador
So, it looks like he was expelled from post shortly after being interviewed by DS agents in September, 2007, then returned a few weeks later, probably to pick up his girl-friend/fiancee with whom he then traveled to Costa Rica to finagle her a visa before returning to the U.S.
Nachman could be sent away to prison for about 10 years. Since friends of his evidently read this blog, I heartily recommend that one of them send him this handy 92-page guide to doing time in a Federal prison. Except that the U.S. Bureau of Prisons does not permit conjugal visits, he should find the experience tolerable. Unless he simply can't resist getting naked in inappropriate surroundings, he ought to earn some time off his sentence for good behavior and could be released just before his 50th birthday.