As a rule we wouldn't recommend buying earrings as a gift. For hygiene reasons they're not returnable, and women tend to be very picky about the earring they wear, making them a very high risk gift.
Quality - If you are determined to go for earrings, then make sure they are genuine Silver and guaranteed Nickel free, as many people are sensitive to Nickel and earrings, being under the minimum weight for Hallmarking (which guarantees that silver is Sterling silver and free from banned base metals) can be a problem if you are not 100% sure of the provenance.
Style - Other than that it's all about style. Does she prefer dangly or studs?
Fitting - Does she prefer loops which dangle from the earlobes or post and butterfly backs which attached the earrings in the centre of the lobe?