I have an iPad Air 5. How do I add a second person (her finger print)?

You can add up to five individual fingerprints to devices that support TouchID. Some users choose to register multiple fingerprints (for different fingers), while others may choose to register fingerprints for different people.

If you choose the latter, consider that no data will be private or beyond reach of anyone for which you have you registered fingerprints. Insofar as the iPad is concerned, being a single-user device, you are the same person.

The iPad User Guide describes the straightforward process of setting-up TouchID - and additional fingerprints.

Set up Touch ID on iPad - Apple Support