Well, if she wears a small: Louis Garneau P-09 Bycicle Helmet Size S NWOT | eBay
Personally, I’d consider the Giro Aerohead sans visor (a-la Wout Van Aert). Not a lot of vents in the shell, but lots of grooves in the foam that allow air to flow inside the helmet (entering from the hairline and out the back). My n=1 is that it hasn’t been hot for me (even with the visor), and I did Nats in it on a day with a 100^F heat index.
