Not during the middle ages, but you specified any time, so, this is the only time I know about specifically; but i do not think foreign observers of wars have ever been exactly rare or unheard of. Anyway, I know there were a lot of foreign, mostly European officers and diplomats, observers present at several large battles during the American Civil War. This was partially to stay current on military technology, as the American Civil War saw the introduction of new types of weapons and tactics. During the 1860's, many European countries had access to contemporary weaponry but there had not been a major war since the Crimean War, so a lot of doctrines were theoretical.

For that matter, a few of the early battles in the American civil war had civilians watching them as though they were some kind of game. It's weird to think about now, but at the beginning of the civil war some people in the North thought it was going to be over very quickly.

Wars in the 20th century have obviously been covered by journalists of all countries, though their actual direct access to the battlefield has not always been permitted. It occurs to me that observing a battle might have been less dangerous in the middle ages (a time when ending battles peacefully to surrender/ransom prisoners to each other was very common) than modern times, with artillery and automatic weapons going off all over the place.

Well, also not in the middle ages, there was an Elean attack on the Acadians right in the middle of the Olympic games (after the pentathlon) in 364 BC. Xenophon writes about it in Hellenica 7.4.29 - 7.4.32. It appears not to have been considered as an intrusion or terrorist act at all by the author, but actually, he admired the Eleans for their efforts.