This is a tough one…it seems really out of place for people to be talking about stuff like that out loud, regardless of how they feel about it. It’s also tough because they’re engaging in some rather self-destructive behavior, and I imagine that, as a friend, it’s probably tough to hear them talk about it.
By the way, if they’re using hormonal contraception (ie: the pill, the patch, depo-provera, etc.), I would suggest letting them know that those methods can cause the death of a newly-fertilized egg, aka a very early abortion (they usually just prevent conception, but they can occasionally prevent a newly-conceived embryo from implanting on the uterus, causing it to die.). Even if they won’t listen, they have a right to know, and you will have spoken for the unborn
Either way, make it clear to them that you don’t want to hear them talking about this kind of stuff when you’re around. If they keep it up, I’d say stay away from them–it isn’t spiritually healthy to sit around and listen to this kind of stuff, and as the previous poster said, a true friend wouldn’t just sit there and mouth off stuff that they knew a friend wasn’t comfortable with. Bad company corrupts good character.