According to her article in thepeerage and limked biographical entry, Lady Henrietta married Colonel John Dyke Acland in 1770 and accompanied him during his posting to Canada during the American Revolution. He was wounded and captured during a battle in the Hudson River valley near Saratoga, New York. She remained behind in Canada, but joined her husband while he was a prisoner of war. They returned to the UK, but she was widowed in 1778. She had a son and a daughter.
From bbc.co; image made lighter and doubled in size by gogm.