Paternal Pride

After you've cleansed Macfarlane's ranch, you'll get the possibility to start the side-mission Paternal Pride, given to you by Bonnie Macfarlane.

She tells you, she hasn't seen her dad, Drew Macfarlane, for days and asks him to go look for him at the barn.

When you arrive at the barn, you'll see the main entrance is closed, so you need to find another entrance. Go left of the main entrance and you'll see a ladder. Go upside and jump under the ceiling, in order that you hang under it. Slide left so that you can pull yourself up and go through the entrance above.

Once entered, you'll see the Undead on the ground floor, doing... nothing. I recommend to shoot them from above, or else, go downstairs and kill them. Beware that it's a small room, because you'll lose the Undead sometimes and they could attack you in the back, on the ground floor.

After you killed all of these zombies, John's going to look for Drew. He finds him, turned into a zombie. You'll have to kill him, just like you did with Uncle. Once killed, you can go back, through the door, back to Bonnie

In a cutscene Marston walks up to her. He doesn't tell anything, so Bonnie knows enough. After saying that John makes her life 'great', full of sarcasm, she goes inside. Mission completed.