Just solved the problem for a work colleague.
Either "alt+blank space bar" or Right click the excel logo in the very top left corner and you get the usual "restore, minimise, maximise" menu but at the bottom of the menu list is an option for "close full screen". This option worked. We found how he inadvertently went into full screen from the "view" ribbon ""full screen button.
We have since found that the "Esc" keyboard button also works as well as right-click anywhere on the screen "close full screen" appears at the bottom of the menu.
