Ms. Lewis was an employee at the time the work was created, whereas Ms. Kirkbride was a stranger to Tesco. There is always an argument that work related to her job created by Ms. Lewis while she was a salaried employee could be assigned to the employer. (This is why lawyers are helpful if you find yourself in a pickle like this, even for a consult.)
As far as the employer is concerned, Ms. Lewis created something for her job, and they own it. This is called a "work made for hire," typically owned by the employer alone. However, it is not always clear who the copyright holder is. From Ms. Lewis' perspective, of course, she created something on her own time related to a part of the game that was not her professional purview. She clearly thinks this is hers alone, and is demanding royalties.
Whether you are an employer or an employee, works made for hire are like quicksand. In this age of immediate, widespread dissemination of content, you must protect yourself with empoyment or independent contractor agreements that spell out very clearly which creative works are owned by the creator and what are owned by the employer. This can cut both ways, and is not just a problem for starving artists. Engineers, computer programmers, singers, and even bookkeepers and consultants need to have clarity before taking on creative projects.
Robust non-disclosure and non-compete agreements can protect both the employer and the employee. It's also important to discuss who has the right to create derivative works based on the product of a particular project. These boring, boilerplate details can become crucial when a product is successful, because they leave a lot of money on the table.