I assume that your friend would agree that it is wrong to take the life of a living breathing human being simply because you do not want that person to be alive. We call that murder and all of us believe that this is morally wrong and reprehensible.
I also assume that your friend would agree that to take the live of a viable unborn baby is also wrong. If a baby is in the ninth month of its gestation period and if that baby would be perfectly healthy if it were born, then your friend would agree that it is wrong to arbitrarily kill that baby, just because we do not want it around.
If this is true, then it is wrong to kill a viable unborn child simply because we do not want him or her to keep on living. This is wrong and sinful, not just according to Christians, but according to nearly all people, regardless of religion (or lack of religion).
Well, what is the age of viability? Is it six months gestation? Is it five months of gestation? On which day after conception do we acknowledge that the unborn baby is actually alive? On what day is it no longer OK to simply and arbitrarily take the life of the unborn baby because we do not want him or her to live? Is it when the unborn baby has a nervous system and a functioning brain? Is it at four months of gestation that we admit that the not-yet-born child is alive and that his or her life has inherent value? What is the age of gestation of an unborn child when we ought to begin protecting it from being killed simply because we do not find it convenient or desirable for him or her to keep living?
Can your friend define when the unborn child is alive? I have decided, upon much consideration, that once a fertilized embryo has become implanted into the womb of its mother is the only point along the path toward birth in which we can agree that this child is alive. Any point after that it is truly debatable and arbitrary to say, for example, that the baby became alive on the 100th day. Because it is immoral to take the life of an already-born child just because we do not want him or her to live, and because it is also immoral to take the life of a not-yet-born child who is, let us say at eight months of gestation, and because there is literally no point from implantation to birth to say for sure that a life has begun, I believe it is wrong to take the life of an unborn child. As a Christian I would never support or agree to the taking of the life of an unborn child.
But, of course, we live in a culture and society which are not particularly “Christian,” so the laws that govern us may not agree with a Christian position, and we need to understand that our society is secular. I am personally opposed to abortion as the killing of a not-yet-born child and I sincerely hope that our governing authorities will see it that way.
People say, “What about rape or incest?” I would imagine that millions of humans alive today are the result of rape or incest. Is it OK to kill them because they are the result of incest? What about if an unborn child at eight months of gestation, who is clearly alive and capable of life outside the womb. Is it OK to kill that child because we do not like how he or she was conceived? This is a very emotional issue and I can see why some people have supported making an exception in the case of rape or incest. I understand this and I want to have compassion for the woman in this terrible situation. I also would not personally condemn a woman who, in her grief and horror at this situation decided to take the life of her unborn child, but I cannot personally support this decision, despite the pathos of the means of conception. But you asked, and, given the reasoning above, I believe it is a sin to take the life of an unborn child.
By the way, there is the extremely rare, but nevertheless real question of the situation where the life of the mother is at stake. In other words, in order to protect the life of the unborn child, the life of the mother will be put at very high risk. This terrible situation, though very rare, is a real thing. In such a terrible situation, I feel it is not my place to make any sort of pronouncement. I can certainly understand that in this case, a medical person is in a deep quandary and the choice to save the mother at the expense of the life of the unborn child, though a terrible decision, may be justified. Medical professionals are occasionally forced to make extremely difficult decisions in which one person dies so that another can live. This is happening right now with extremely ill Covid-19 patients and the available space in some ICUs. The Bible does not address this question directly, but I believe that we can let medical professionals help parents in this case make the best moral call when it comes down to the terrible situation of choosing who will live. It is not my place to prejudge such a situation.