Personal data
Friederikke of Saxe-Weissenfels
Born July 17, 1715  
in Gotha
Died May 12, 1775  
in Langensalza

Daughter of Frederick II of Saxe-Gotha-Altenburg (1676-1732) and Magdalena Augusta of Anhalt-Zerbst (1679-1740), and older sister of Augusta (1719-1772), who in 1736 married Frederick, Prince of Wales (1707-1751) and in 1738 became the mother of George William Frederick (1738-1820), the later King George III of Great Britain. Married (1734) to Johann Adolf II, Duke of Saxe-Weissenfels (1685-1746), with whom she had five children, none of whom survived past the age of nine. 

Boswell, posing as a Baron, was invited to the court of Friederikke at Langensalza on October 21, feeling "quite at my ease in the house of the sister of the Princess of Wales and the Aunt of my King."

    Fredericka of Saxe-Gotha-Altenburg (Wikipedia)
    Boswell on the Grand Tour I, 1764

    Unless otherwise noted, the source for a dated quotation of Boswell's is generally the corresponding volume of the Yale Trade Editions of Boswell's journals.