From Zeepvatt's bio on Leopold: describing F at 25 'a tall woman, stately rather than beautiful, and quite shy'
'In October the Queen allowed him to visit...Gmunden, officially to see the widowed Queen on her behalf, but really to assess his chances [for marriage to F]. Unfortunately, this only deepened his feelings for another woman who was out of reach. Frederica was in love with her father's secretary, Baron Alfons von Pawel-Rammingen. The old King would not hear of marriage, nor would his son, and she would marry no one else. Leopold did not propose because he sensed that it was hopeless..'
'After his failure to propose...[he] felt very dispirited. In January 1879, she arrived in London, and he was sent to meet her. He found it hard to shake off the longing that thigns could be different between them, and as the weeks passed, that longing grew. In February..he told his mother that he wanted to marry and she was sympathetic, though she advised caution. Frederica was not mentioned...The dream of Frederica was doomed to failure. Leopold still hankered after her when he returned...in april, and Alick Yorke appealed to friends to speak to her on his behalf, but soon Frederica took the matter in hand and confessed her feelings for Baron von Pawel-Rammingen. To his credit, Leopold's attitude changed as soon as the truth came out...he set his feeings aside and did his utmost to help her. As a member of the House of Hanover, frederica was also a Princess of Great Britain and Ireland, and the Queen could give permission for her marriage. This she did, undertaking to pay for the ceremony and provide the couple with a home. 'Leopold has been her great help throughout', she told the Crown Princess [Vicky].'
As a mark of his friendship, when he married Helen, knowing his time could always be cut short, he had included a bequests of GBP 1000 for Frederica who 'had no resources of her own and remained one of his dearest friends'.
Frederica in turn became close to Helen as well, frequently visiting her and Leo. This friendship continued, perhaps even strengthened, after Leo's death.