Why did my friend get cervical cancer?

A friend of mine in the last few years has won the battle with cervical cancer. Although cervical cancer is often thought to be caused by HPV, smoking, or prolonged use of birth control, she seemed to be an exception since both she and her husband were always monogamous, neither smoked, and she never used birth control- rather, she had always struggled with infertility. However, a couple years after her cancer, we discover that her husband has been involved in questionable activities for at least a year or two, possibly much longer. So I wonder, what is the chance he was unfaithful after all a number of years back and he infected her? I am 99.9999% certain she never was unfaithful, or engaged in premarital or extra-marital . Or is this whole thing just a giant coincidence? What do you think?