What you were told is correct.* You should never change the grade around a tree.* Willows being the heavy root growers can deal with it a bit better then most, but for the health of the tree it would be best to expose the rootflare.* You can trim any feeder roots that have risen to the surface looking for oxygen.* Take a look here with some videos you can view that will be helpful.* Lots to click on at the bottom of the page too.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.