there is not a way to turn off autofocus on the front camera since its designed for auto -selfies. auto focus is a system feature.
if she wants to LOCK her face so the sensors focus on HER face, she can tap the screen where her face is and it should keep her face in focus.
for serious video your sister should use the rear camera. enable auto tracking.
if she MUST use the front camera there are things she should do to prevent the camera sensor from "hunting" for things to focus on, unless she wants the sensor to hunt for focus.
but to directly answer your question, there is no way to disable that on a stock non rooted camera.