Mandy was the daughter of one of White Orchard's residents, Odolan. With the war going on in 1272, the local drinking water became contaminated with dead bodies and Mandy, having drank this water, became very ill. Her father was informed the only way she'd get better was to find a clean source of drinking water.
If Geralt helps Odolan: Mandy survived thanks to the witcher killing off the Devil by the Well.