In a relative sense, sure, there is a VERY slightly greater risk of breast cancer in having the abortion AS COMPARED to having the baby. The abortion would keep the status quo/default level of risk she was born with intact. Giving birth would slightly LESSEN that "default" risk level, though probably not enough to even make her reconsider.
One of the biggest problems with the ABC argument is that it appeals to the selfish nature of shortsighted young women. It's not about right or wrong, it's about the girl and some infintesimally small difference in risk to her breast health when she's 50. First off, she doesn't care. She just wants her baby dead. Secondly, even if you manage to scare her off the abortion track and save the baby, you haven't changed her mind about abortion. You've just lied to her and made her fearful.
This is just a desperate scare tactic, and it does a real disservice to our cause. We are in the right, and time and hard work will bear that out. We WILL win--but not by twisting the facts and sullying our trustworthiness.
I've tried to tell them this before, but I just get berated and told that I do not know what I am talking about.
Though, it does knock a bit of a hole in the Pro-abortion argument that abortion is so much safer than having a baby - which is the baseline understanding by many.