"What is her proof?"
Translation:¿Cuál es su evidencia?
My understanding is that "Qué es...?" means essentially "What is the definition of...?" Unless you are asking for a definition, you will want to use cuál.
You should use cuál and not qué for this sentence.
The answer to "¿Qué es su pruba?" would be:
Her proof is the convincing information that she has about the issue. In other words, Qué is mostly used to request a definition of a word. At least this is the official grammar book usage.