Shawls certainly are different than sashes, so I wouldn't think there are any specific "rules" or conventions governing how they should be worn.

My wife wears a shawl on both shoulders, when she wears one. She frequently wears a pashmina, which is similar to a shawl, again over both shoulders and knotted somehow in front.

When she wears a silk sash to balls she wears it over her right shoulder, crossed over to her left hip, then back around to her right shoulder.

All this being said, I wouldn't dare suggest how your friend wears a tartan shawl! However she thinks looks best, or keeps her warmest, or both, is the best way.