it only would have been on wifi IF wifi was turned on. you can use data on an iphone without wifi being activated.
her phone is always on wifi it automatically connects when she is in range of the network just not sure why she is getting data unless she wasnt home
That could be anything really. Delayed data usage report, updates that have to go over the cell network, she could have took her phone to the bathroom haha.
Multimedia messages or iMessage
Its also possible that her phone had the wifi setting off so she tapped into her own data usage instead.