Could be a multitude of different things below, I'd need some more info to make a better guess. Did the temperature gauge spike up into the red, or did she just notice steam coming from under the hood while the gauge stayed normal?
- Radiator
- Coolant
- Electric cooling Fans
- Water pump
- Thermostat
- Hoses and clamps
- Transmission oil cooler
- Coolant pressure bottle/overflow system
Based on what she described, if the gauge reads hot as well it could be due to a coolant leak, blown water/coolant pump, bad thermostat or faulty radiator fan due to failure or a belt breaking.
If it's the latter, it could be a faulty temp sensor. Essentially, if the sensor is broken and thinks the engine is running cold it will not allow what it perceives to be "excess" heat to vacate the engine compartment.
Good luck with the repair, hopefully it gets fixed under warranty, and let us know what it actually was. (My money's on a temp sensor, water pump or thermostat).