american soldier in nazi concentration camp?

The incidence of Americans in the KL is VERY RARE. It is true there were a few held in Buchenwald, but not because they were Jewish. In fact, they weren't. Buchenwald was not a death camp, though many inmates died there. It was essentially a camp for political prisoners, and there were many common criminals amongst them. The camp rolls are littered with "famous people" from all over Europe, however, I doubt very much you'd find American Jews amongst them, especially American Jews who were part of the military.

The scenario of his capture could be anything, starting with the airborne drops on the night before D-Day and continuing on through the Battle of the Bulge and nearly all the way to the end of the war. Mel Brooks, a rather famous American Jew, was captured during the Bulge, for example. For the most part the German military, both SS and Wehrmacht, didn't have time to worry much about the Jews they happened to capture. They were simply herded into POW pens and then moved about as the front moved east. Remember, the Luftwaffe had many many American and English Jews that they held for more then 4 years, and nobody bothered them much, partly because the military didn't really buy into the whole "Jews are our enemy" thing and partly because they were covered under the Geneva Conventions, which the Germans generally respected.

Near the end of the war you MIGHT be able to make a case that some Americans got better treatment, simply because once it became obvious the war was lost, almost everyone wanted to surrender to the west and NOT to the Russians. However, there is very little factual data to support that idea (better treatment) and for the most part they were just treated the same as the British. The Russians were a different case, but that's a much longer story.

There are many many volumes regarding the treatment and conditions in American POW camps, or the camps that predominantly American. Most of the books detail the camps for fliers, simply because they were there longer. Remember, other than fliers there weren't any Americans fighting in western Europe until 1944, so they were only there for about a year before the war was over. You can get some ideas about liberation of the camps from The Last 100 Days, although those were all Luft camps. There are a number of first person diaries available also, and a quick Google search will find them. Here's some general information.

http://www.historyonthenet.com/WW2/german_pow_camps.htm

Now, as for them being able to meet again, the water is murky. Assuming she's a military nurse, which is about the only way she could end up in Europe at the time, she's going to be assigned somewhere. Likewise he's going to be assigned duties once he's freed. Although it could happen, them meeting up again is unlikely. Certainly strange things did happen, but they are both unlikely to know anything about where the other is, and the military isn't going to spend too much time helping them find out. There were more important things to do. Once he is freed and processed, he might have been granted some leave, but transportation would have been limited, at least within Germany, and, again, he likely wouldn't know where she was. She might know he was alive and captured, but it's very unlikely she would know where he was found when liberated. Highly unlikely either of them would know an APO address for the other. Could happen...but it's a big stretch.

BTW, if you were to post this on the WWII thread you'd be more likely to get additional ideas and answers. New threads at this time of year don't get monitored, unlike the existing threads.