In the Spring of 1272, Rhosyn fought in a battle near White Orchard, where he twisted his leg, and attempted to crawl away from the battlefield to safety. In the process, he encountered a wounded Temerian soldier named Bastien, and the two helped each other find refuge in a nearby hut in the woods. They sat there for several days waiting for help, as they were unable to move anywhere due to their injuries. To pass the time, they'd tell stories to each other, like the time Rhosyn's commander once demanded his soldiers obey his every order, including "shitting on command."
If Geralt decides to help Dune Vildenvert find Bastien, they encounter Bastien and Rhosyn in the hut after being led there by Bastien's dog, Hussar. Bastien refuses to abandon Rhosyn, while Dune initially refuses to take him in due to his deserter status, as the whole family could be punished if Rhosyn is ever found to be in their fold. Ultimately, Geralt must make the choice of whether to agree with Dune, or persuade him to take Rhosyn in.
If Geralt tells Dune to take him in: everyone, including the woman Dune mentioned and the dog, can be found at their farm just east of White Orchard village.
If Geralt tells Dune it's too dangerous to take in Rhosyn or finds the cabin before talking to Dune: Rhosyn dies.
If Geralt doesn't do the quest: Rhosyn and Bastien both die.