The footer, which is displayed on the website, is always from the Theme (probably footer.php).
My question is: is there a way to change what is displayed in the footer, by means of a plugin? (I could just go and edit the footer.php of the Theme, but my boss doesn’t want that, he needs the flexibility to change Themes as he wishes)
Educate your Boss, that’s one of the benefits of child themes. ;D
There are many plugins that will let you insert into the footer, but if you really want to control the look and presentation then editing a child theme copy of the footer.php and the child theme’s style.css file is the way to go.