Cervical cancer isn't "transmitted" by sex at all. Cancer of any sort can not be transmitted.
HPV (human papillomavirus) is the virus known to be a cause of cervical cancer in some cases (NOT in all cases, just SOME). HPV is what gets sexually transmitted - and even then it's more likely to manifest as genital warts. Most people with HPV will not get cancer as a result, and most people with cervical cancer either don't have HPV or have cervical cancer caused by other factors.
The biggest risk factor, as will all cancers, is age. It's certainly not a cancer seen in teenagers.
Both you and your friend are wrong: she does not "know" she has cervical cancer (far too young), and you do not know that cancer can not be "transmitted" by sex. The HPV can be transmitted by sex, but in most cases it isn't the cause of cervical cancer.
ETA: poster "nochocolate" should know that the vaccine is for HPV, not cervical cancer. There is no vaccine to stop you getting cervical cancer. You can get the vaccine to stop HPV, but since HPV isn't the primary cause of cervical cancer, it doesn't really reduce your chances of getting cervical cancer later in life.
I am also doubting "Matthew" status as a Medical Student...shouldn't he know that HPV isn't the primary cause of cervical cancer?