first, i would celebrate with her that her baby is well and healthy enough to be home. this is cause for celebration and you should be happy with her.
second, i would not be surprised if she is as worried as you are, if not moreso, about her child's health. if she even mentions it, offer your support in whatever capacity that she needs. A simple--whatever you need, i'll be there for you as best i can--will suffice.
third, if you get into longer discussions about treatment options and what not, may i recommend that you share information about this amazing treatment facility? The Family Hope Center
is an AMAZING organization that helps heal brain injuries and defects, usually non-medically (even though it's run by neurologists!). people come from all over the world to heal their children and themselves. i have seen babies go from blind, deaf, and non-moving and most doctors saying that there is no hope for a normal life, to them being sighted, hearing, speaking, walking, and working at grade level.
this process works! there is hope, even if the little one is injured. Please, keep the Family Hope Center