Thank you very much for the question. In order to explain this Gemara, an introduction is needed. The Gemara in Bava Kama 60a discusses saving oneself from death by using or taking someone else's Mamon. There is a dispute among the Rishonim whether it is permissible for a person who is in a state of Piku'ach Nefesh to save himself with his friend's money. Tosfos and the Rosh maintain that it is permissible for a person to do so, since there is nothing that stands in the way of Piku'ach Nefesh, but the money must be returned afterwards. Some Acharonim (see Teshuvos Binyan Tziyon 167:9 and others) understand that Rashi's opinion is that it is forbidden to save oneself with the money of one's friend even if the person is in a situation of danger.

According to Rashi's opinon , the Binyan Tziyon explains the issue here as follows. Rebbi Yehudah ruled that it is not permissible to use someone else's money in order to save oneself, and Rebbi Yosi disagreed with him and rebuked him for robbing the shepherd. After that, when Rebbi Yosi took food from all the citizens of the city, and with permission, Rebbi Yehudah said to him, "Just as it is permissible for you to take from the citizens of the city, so it was also permissible for me to take from the shepherd without permission, since I maintain that it is permissible to steal in order to save yourself from danger." That is, Rashi's opinion mentioned above is only according to Rebbi Yosi, and as the Gemara mentions in several places, in a dispute between Rebbi Yosi and Rebbi Yehudah, the Halachah follows Rebbi Yosi.

(By the way, there is a lot more to elaborate on in the Sugya of saving oneself with Mamon Chaveiro, and it is a very interesting subject.)

Others (see Benayahu here) explain that Rebbi Yosi and Rebbi Yehudah said the things in jest, in order to make each other happy when they were weak after being sick.

Best Regards, and Happy Chanukah,

Aharon Steiner