i realise this is a tough time for all - just a little aside - she isn't downs - she just happens to have down syndrome - it really isn't the sum total of this wee girl -as you will learn in time . If you have a look at my profile you will see pics of my dd1 who was born with down syndrome. Some baby pics and her latest pics - she is now 5 yo.
They will be grieving as they have had a loss - the babe for whom they dreamed and planned for is not what they expected and so new dreams and plans must be made BUT we need to farewell the old ones iyswim. They will be in shock, they will be angry, they will be upset, they will even go through a phase of blaming themselves.
what can you do?
first and foremost - whatever you would do for anyone who has had a new babe - celebrate the amazing new life that has arrived. Celebrate with her parents for being parents of another gorgeous girl.
perhaps contact Down Syndrome Association UK - they have fabulous phone support as well as send out new parent kits. I think you could seek out information for them - something they may not feel ready to do but you can juse have it ready for them.
be there to listen - not to comment or judge or anything - whatever they are feeling is what they need to be feeling at the time. Just be a shoulder to cry on and an ear to listen.
Practical help - in whatever form you can manage - will face some tough days ahead.
I am only sharing what I have witnessed as a new parent support person - unlike most people we knew before dd1 was born and having had positive experiences with families who had children with down syndrome it was not a huge shock - that is not to say we did not have to adjust our mindset and dream new dreams for her ( such as olympian sprinter - she can run like the wind, rather than neurosurgeon) For us - she was our babe and was to be born - as you said for your niece.