I had the same problem for several years. Their smoke would naturally rise up and into our open bedroom window on the second floor, We could barely walk into our back yard on some days as the tobacco smoke would be so thick. I ended up buying a 20 inch high-velocity floor fan from Home Depot and placed it on the concrete patio aimed up over the fence so the smoke would blow up and away. I used a wireless remote switch to turn it on and off from inside the house. Worked great in our situation. I had to bring it in during rainy season (not much of that in California) but they would move their smoking to their garage during the Winter anyway.
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Wireless remote for fan
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