What is the penalty for stealing an individual car keys and driving her car
Without a license while that individual is hospitalized?

Where are you located ?

Grand Theft auto penalties are up to local laws. Also it will depend on a number of other factors. This situation doesn't sound like car theft because it sounds like the car was returned. It sounds more like unauthorized use which may not even be a criminal act. Sentencing also depends on the record of the perp.

I'm sorry, but there is no black and white answer to your question. A little more background might help us help you.

Also... has this been reported to the police yet?

It is auto theft, does not matter, how you get the car, went into a parking lot and hot wired it, or stole their keys.

The actual charge will have different names, grand theft, theft by taking, felony theft, and so on. Since it did not involve violence, most courts would give less than 3 years to serve in prison.