You have an advantage no one else has: you can see what she wears and what she doesn't. Use it.
The pieces that come out every day are the brief. Note the metal, the size, and whether she layers or wears one thing at a time. Buy inside that brief. The most common mistake is buying something louder than she is, because it feels more like a gift.
Should I get something that matches what she already has?
Matching is safer than surprising. A second chain she can layer with the one she wears, or earrings in the metal of her everyday ring, will be worn from the first day. A new style she has never tried needs her to decide she likes it first.
Is it better to give one piece or a set?
One piece, chosen well. Sets look generous but usually contain something she won't wear, and that piece quietly devalues the rest.