Fionn the term queen mother has a dictionary definition of
"the widow of the former king who is also the mother of the reigning monarch"
otherwise she would be the queen dowager. It has been used throughout history, most recently Queen Mary of Teck, the widow of George V and mother of George VI and the earliest reference I found was Eleonor of Aquitaine wife of Henry II and mother of Richard the Lionheart - I wouldn't have fancied your chances telling her she wasn't a Queen!!!!