There was a big age difference between Fiona Thyssen and Alexandros Onassis. That was the main reason she was initially reluctant to become involved with him, Fiona was old enough to be his mother. He pursued her relentlessly however and they ended up falling deeply in love. They were living together in Switzerland at the time of his tragic death and were planning a life together.
In the book Nemesis
by Peter Evans(a truly shocking read in too many ways to discuss here
) Fiona asserts that both of Alex's parents Ari Onassis and Tina Livanos Onassis Blandford Niarchos loathed her and moved heaven and earth to end the relationship. Onassis hated her because he wanted his only son to marry a young Greek shipping heiress who could continue the Onassis name-and Ari might also have fancied Fiona for himself according to Thyssen.(in other words sexual jealousy)
As for Alex's mother Tina, she spread the word that Fiona was a high end call girl in order to turn her boy away from this glamorous much older woman. :ohmy
Nothing worked. And in the end the loss of their only son psychologically decimated both Ari and Tina, and almost certainly hastened their physical declines and deaths soon after.
Fiona also sort of went into a decline in the years after the death of Alexandros . One gets the feeling that this much younger man was the great love of her life.