I've never had a problem with soot but when I was a flower courier there was one lily that had very heaven brown pollen. The only way to remove it if you got it on your clothes was with sticky(Cello) tape. The sticky tape would lift the pollen right off the clothing. If you tried brushing it you got a terrible mess that wouldn't wash out. I would then try the Dawn detergent. Dish liquids are made to cut through grease. Best of luck to your friend.