1. Dear Olivia, the first thing you need to realise is that in law, you own the copyright in the original files from which the photographs were taken. Essentially, as you created the original files, copyright in them attaches to you as photographer and not in the persons whose image was taken. Accordingly, you can refuse to release the original files to your unhappy customer if you want to retain the images. This is because you created it and hold copyright in these original files.
2. The term for which you hold copyright in these original files is the duration of your life plus 70 years. So, you don't have to release them to this unhappy customer. Additionally, the original agreement whereby you took the photographs would give you a general licence to have taken the photographs. So, you haven't infringed the original agreement which gave you the licence to take these photographs. If you require any further clarification, I will be happy to help further.
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4. Well, in law, the client can sue you for the original files. However, she will quickly get advice that her claim will not be upheld. Her remedy would be to argue that the files cannot be published as she has "image rights" in the photos. However, this still does not give her the rights to the original files. However, if she does slander you, then you can sue her for defamation. However, that is like using a sledgehammer to crack a nut as it does not look good to customers that you sue someone for them complaining about your services.