What is the weather term for snow and dust mixture?
I lived and flew in North Dakota for about three years. While I was there, people talked about (and I experienced) the delightful mixture of snow and dirt called snirt. It can be the result of plowing sanded roads, and also can be caused by the high winds that come out of Canada blowing dirt from fields onto areas still covered in snow. Google image search has some good examples.

I have a feeling that's what was described by your friend. It appears in Wikipedia's list of types of snow, but as far as I am aware it is not an FAA or NOAA term.