First of all we need to see if there are any indications of her breast ever being affected in her life? The breasts are represented by the Moon(the mother, the nurturer) 4th house, and planets in the sign of Cancer.
In her chart the Moon is with Mars, which rules invasive operations, knifes, cutting, accidents, damage. And in it is opposed by Pluto-the planet of total transformation, surgeries, annihilation, mutation, plus another difficult aspect from Saturn-the Great malefic, which is also connected to malignancy. Pretty battered Moon as we can see, aspected difficultly by most of the baddies as we call them in astrology! Pluto and Saturn both are connected to malignancy and mutations in the cells, so her breasts and other baby producing organs(ovaries, womb, etc) are much more susceptible to cancer like her damaged gene showed. The Moon also rules her first house—the body overall, so her whole body is hammered by these malefics, therefore she needs to be extra careful for wear, tear and degeneration of her health.
4th house is the other place which shows breasts and it has Uranus and Pluto there- so called malefics, which create, sudden changes, mutations and crisis like situations. They have disruptive and abrupt influence. . The sign Cancer(also rules breasts) is where Agelina has Venus-the most beautiful planet, which bestowed her with wonderfully shaped breasts! Many women with planets in the sign Cancer can have prettily formed breasts or some emphasis on the breasts. But Saturn, the great Malefic is also in Cancer, which casts a shadow over her pretty Venus bosom!
Why now exactly? Because her moon which is by birth so heavily afflicted, is triggered by Uranus and Pluto transit—the most life changing planets ganged up on her Moon (breasts) together. Pluto and Uranus were also triggering during the operation her Mars, next to the Moon, and Mars naturally rules operations, invasive procedures.
It is so clear and simple once the event has happened, but I guess few astrologers could have predicted exactly what will happen. Many would have seen that her health your be in jeopardy due to these transits to her Moon, which rules her 1st house of body and that it would have been most likely the female organs.
But no matter what, I think that she was indeed of a grave danger to develop cancer, due to the birth aspect of Pluto(mutation), Saturn(malignancy) and Mars(damage) to her Moon(breast) so she took the right decision right before Pluto and Uranus could trigger her Moon more seriously for 2 years and possibly trigger the Cancer!