Alternatively, she can be sold to Grisham in exchange for 100 meat jerky after three months and a loss of 5 Karma. If the Chosen One changes their mind, Bess can be released along with the rest of the brahmin in the slaughterhouse pen for 10 Karma, but their reputation with Modoc will take a hit. Bess also can be a temporary companion after being helped in numerous instances.
Bess will disappear forever if the Chosen One goes down into the Modoc caves (this includes the well and outhouse).
It's actually possible to have Bess join the Chosen One permanently throughout the game. After she has been slaughtered by Grisham or after you have had the Bess encounter you have to return to a point in the game where you haven't healed her broken leg yet. This can be done by temporarily removing the file Modocmain.sav in your current savegame slot and replacing it with one that was made prior to healing Bess' broken leg. In this file variables regarding Modoc's main street are saved after visiting the area. When you now heal her leg again, she will follow you for good. You can even initiate dialog with her, allowing you to alter her combat options.