Sorry you didn't get a quick answer. Yes, dust will stick to *fresh/wet) royal icing but if the icing has already dried you can very lightly dust it w/a slightly damp/wed art brush before adding the dust.
I think what she's asking is the other way around. She has a hardened flower that has been dusted and now she wants to add a royal center and is afraid that the royal center won't stick to the dust already applied to the gumpaste petals. I don't think it will but it depends on how heavily it was dusted.