You have to go to the Horse Lake NERO Checkpoint. If you have cleared it before, you just need to Fast-Travel there. There’s also a Horde that roams the main street up to the train near the pond where you need to gather lavender so make sure to keep low and look around before collecting the plants.
The plants are all located near the shore, a few meters away from each other so you don’t need to venture far to get all of them. If a handful of freakers notice you, draw them away from the Horde before dealing with them. Once you have three lavenders, head back to your safehouse and approach Boozer for a cutscene.
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