Well, thankfully NOT 100% of them die before their first birthday! The likelihood is that yes, this baby will not have much time before passing on, but I learned an awful lot when two years ago my husband and I sat across the room from a team of drs while they speculated what all was wrong with our baby. Though our baby's final diagnosis did not involve trisomy 18, he had a number of different conditions and malformations that gave us a 60% chance of him surviving the first month, *if* he lived through birth. While we are living a miracle and he is now 18months old and doing worlds better than anyone could have ever imagined, living through the fear and stress of that pregnancy was some days extremely difficult. Just tell her you love her and are thinking of her, and that you are praying for her. How far away are you? Are you within a couple of hours so that you could go and visit her if she was up for it? Some days I just needed someone there so I could go out to lunch and life would feel normal...and we could talk about normal things and not have the conversation revolve around the latest dr appointments and diagnoses. It got hard to have to explain things over and over, and the chance to just be with a friend was a nice distraction for a few hours. If you are close enough to visit-make sure she's up for it, some days I just didn't want to be around anyone.
Like someone mentioned earlier, don't try and come up with magical words of wisdom, sometimes they just don't quite hit the mark, even when they are said with the very best of intentions. And honestly, don't tell her "I could never go through this" or "I can't imagine how strong you must be to deal with this"...I know everyone who said that was trying to be encouraging about the way I was handling things, but really, while there were some days I know I was carried through my fears and frustrations and sadness solely on the prayers of others, there were some days when I felt like I couldn't go through it either. Oh, and I assume she's married...don't forget about her husband! Everyone flocked around me because I was the pregnant one, and my husband-who was scared just as much about what was happening-was assumed to be dealing just fine with everything because he was the man. He was hurting too and shed plenty of tears himself, but nearly everyone forgot about him. If you end up sending anything to them, flowers or whatever, adress it to both of them, not just to her.
Keep it simple.
And don't forget about her after baby comes, whether baby only survives a few hours or days, or whether baby is one of the few that has a longer life. Everyone is thinking about her and her family now, but in a few months life will go back to normal for everyone and she will still be reminded every day that she doesn't have her baby in her arms. Mark your calendar for a few weeks or months from now if you need to to help remember, and give her a call then too, just to remind her that you love her.