Start this mission off at the yellow marker on the map, and Bonnie will tell you how worried she is about her father. Follow Bonnie on your horse until you track her dad down in the outback, who’ll ask you to head back for a carriage.
On your way back, you’ll notice a fire in the distance! Charge down to the burning barn and jump off your horse. You need to climb up the back side of the barn, up and around onto the windmill platform (use Y/Triangle to pull yourself up) and hang off the wooden pole on the barn.
Now shimmy across to your left, run around the platform and into the barn, where you need to slide down the ladder, pull the pitchfork out that’s blocking the door and then slap the horses on their rears to get them to leave. The final horse won’t be quite so co-operative, so you need to mount it and ride it out of the barn (press jump before the entrance to clear it) to complete the final main mission in Bonnie’s story arc.