My biggest advice is just to be there for her and to listen - really listen - when she needs to talk. It is perfectly acceptable to offer to watch her daughter, to offer to go to appointments with her if she needs support, let her know that if she needs to go to the hospital, she can drop her daughter off to you any time of the day or night, etc... It is also thoughtful to offer to do errands or shopping for her, drop off precooked dinners, etc... if you are willing to do that. To say her world has been turned upside down is an understatement. Parents go through periods of anger, denial, fear, grief, etc... when they learn their child has a disability. I had two high risk pregnancies (and two miscarriages) so I can tell you it is very, very stressful to not know if your baby will make it or not. My oldest child has autism. So I know what it is like to find out that my child has a disability and then live with it.... But I didn't get that piece of news until he was 3 so it wasn't like I worried about it all during my pregnancy (I was too busy worrying about all the preterm labor anyway and the amnio that said he had markers for downs syndrome - which he doesn't have). Even once the baby is born, I'd expect her to have ups and downs. The grief comes and goes. And if she miscarries, expect that she'll need lots of support. She'll get lots of "it is for the best"s but not much genuine sympathy and support. That baby is very real to her and she loves it no matter how the pregnancy ends. Genuine sympathy and letting her grieve and vent her anger, sadness and grief is really a gift. Losing a child is never easy and it never feels like a good thing no matter what. Raising a child with a significant disability is not easy either. You end up feeling very alone and very isolated. Your friends don't understand and they have their own lives and tend to not be very sensitive (in my experience). A simple "I'm here for you" is a very good thing to hear - especially if you mean it.
All that said, I work for Early Intervention. Which means I work with kids with developmental disabilities from birth to age 3. Lots of us that work with EI have a loved one with a disability. That's how lots of us get interested. Your friend should start looking into early intervention where she's at. Services can start as soon as the baby is born. I'm a developmental specialist and I work with occupational therapists, speech therapists, feeding specialists, physical therapists, nutritionists and the list goes on. We provide support and services for the family as well as the child. It is a free service and you can refer yourself. It also gives you access to support groups and information.
At this point I've seen quite a few babies with genetic diagnosises and it is true that you really don't know what you are going to get until the baby is born. Some are very badly affected and others much more mildly affected. If she isn't seeing a perinatologist (maternal/fetal specialist), she should get a referral to one. Their knowledge and experience is much more extensive than a regular OB when it comes to a complicated pregnancy. She should also decide where she's going to deliver her baby. She'll want the highest level NICU she can find at the hospital where she delivers. If there are complications, she wants access to the doctors and equipment of a high level NICU. It wouldn't hurt to talk to her pediatrician now and see if he/she is going to be a help after the baby is born or if they need to find a different one. A perinatologist should have access to a geneticist and or a genetics counselor. I read a little about Turner Syndrome but obviously I'm no expert. But in my experience, kids with genetics conditions tend to be less affected if they are mosaic (genetic defect only in a portion of the cells) as opposed to not (genetic defect present in all cells). I wish your friend the best of luck. She's at the beginning of a very hard road no matter what happens from this point on.
T.