Wide-ish tires with a smooth tread at a pressure appropriate for your weight have less rolling resistance, generally. Beefy tires with a lot of rubber, stiff sidewalls, and low threads per inch roll poorly, regardless of width.

If she's under 200lbs, 32c tires would be about perfect. Vittoria Hypers are a good "performance" commuting/recreational tire. Vittoria Randonneur's are my preference, though.