I am sorry to read about your friend.

Be there for her - she will need all the support she can get.
Having been down the TTTS road myself I know that nothing is guaranteed. She is pretty sure that she will lose one twin but there is nothing to guarantee that the other one will survive. So please don't put too much emphasive on the one they expect to live.
I t is still very early days yet. I was told that before 28 weeks things wouldn't be good for either of my girls. But so many things can go wrong with this condition and if the worse happens she will be verys grateful to such a kind and considerate friend as yourself.

Love and my best wishes to you and your friend.
Debbie