Geralt met Aldert during the May of 1272, at the tavern in White Orchard. Unlike the rest of the populace, he seemed eager to reach the front lines of the war between the Northern Kingdoms and Nilfgaard, in order to describe and catalogue it in what he hoped would be his magnum opus, a text describing the third Northern war.
Aldert was also an avid gwent player, using the Nilfgaardian Empire deck, which complemented his interest in the war. He may have taught Geralt the rules of gwent, and, if beaten, gives the Zoltan Chivay gwent card.
After Geralt left White Orchard, he may have found Aldert hanged at the Hanged Man's Tree in Velen. Beneath his body, you can pick up and read the book he was writing, and the gwent card he had if you didn't beat him in White Orchard.