The dress is fine, but the sandals need to go. She would look better with flats or heels. The color of her nails should go with the secondary color of the dress. Since there is gold on the dress, her nails should be this color as well. As for the shoes, she can go with either the primary color black, or the secondary color gold.
She CAN.. But in my opinion the dress is a little more fancy than the sandals so I personally wouldn't really match them together. Also black nail polish would make it too much black on black for me. I would do a gold or nude.