Sash on the left???

[Dollander]

I always thought the convention was that royalty/ nobility wear their sashes (orders, whatever) on their left shoulder, while regular folks will wear it on the right. And the Lord Lyon's specific recommendation about tartan sashes is that only if a woman is either chief of her clan or married to the chief is she to wear a tartan sash on her left shoulder. But like Steve pointed out, his authority only pertains to arms, not the conventions of Highland dress.

I suppose it's not unlike stepping on someones's toes by wearing a purely decorative hackle in a color that just happens to represent a regiment in which you never served. And seeing as you live in a country where titles of nobility are nonexistent, unless you're really, really worried about someone getting offended because they think she's posing as the chief of Clan Douglas, I'd say she should wear the sash whichever way you both think looks better. Just my two cents.