I must correct one point of Melanie's response - Down syndrome is a chromosomal abnormality, present from conception or just after conception, where there is an extra #21 chromosome. Having fluid on the brain would not cause an extra chromosome to be present in the cells, but perhaps what she meant is that it could result in developmental/cognitive delays similar to what one finds in people with DS.
I believe the usual treatment for hydrocephalus is to place a shunt to drain off excess fluid into the body where it can be eliminated easily.
I recommend they take someone with when they see the specialist, someone who can be another set of ears, because when faced with challenges like this, people often don't retain all the info and need to hear it multiple times, or can't think clearly enough to ask questions.
Also, I suggest they find other parents whose children have been born with similar issues and talk to them to get an idea what the reality could be. There may be parents who have agreed to be mentors or talk to new parents. If in SF, I recommend calling Support for Families of children with Disabilities for information and connections.
I am sure it is a very hard time for your whole family, and my heart goes out to them. I know from my own experience that you have to grieve the loss of the baby you thought you were going to have, but it doesn't mean the one you have will be less loving, lovable and loved.