In the early 1920s, she established a close friendship with Ivor Novello, who was then a young, handsome actor. Around the same time, Collier became seriously ill from diabetes. First sent to Switzerland for treatment in 1923 then taken to a doctor in Strasbourg, she was the first patient in Europe ever to be treated with the drug insulin following its recent discovery by Frederick Banting and others in Toronto. Fully recovered, she played the Duchesse de Surennes in Somerset Maugham's comedy "Our Betters", and in the following year, collaborated with Ivor Novello on a play called "The Rat". She also appeared in several plays with him, including the British version of the American success, "The Firebrand" by Edwin Justus Mayer. In 1924 Collier introduced Novello to the poet Siegfried Sassoon, with whom he had a six-month affair. While Sassoon destroyed his diary for this period, his unhappiness over their relationship became well-known, and according to his biographer Jean Moorcroft Wilson, 'pitched him into he referred to as an "unblinking hell"'.