A demo CD for vocal artist / singer is typically a recording of song(s). If your friend has done voice over, then the demo CD should contain something she has done already. Or it can be something specifically recorded for the purpose of making the demo CD.
The demo CD should contain works of your friend, which should help the potential reviewer to evaluate her ability for a particular task.
If you have doubts, don't hesitate to ask them what would they want in demo CD.