Following on from FumbleFingers' comment,
the pizza is hers.
To say that it was her would indicate that she had been turned into a pizza rather than being the person to whom it belonged.
PS. I recently, with the approval of the Portuguese snack-bar owner concerned, scratched out the apostrophe in the sign on the toilet door reading: Customer's only - ...
As a general rule, "who" should be used for people. "Which" is used for things. With animals, it depends on context. Animals are often anthropomorphised in literature, and also in daily life (especially pets), and so may be referred to in the same way as a person.
Some examples in media referring to a specific animal by its species, not by its name, but ...
Some who dislike the singular they might argue that in your sentence, they refers to the people who operate the restaurant. But in practice, they means the restaurant as an indivisible entity, not the collection of people who own, manage, and staff it.