I agree with the other posters – just tell her you’re sad for her and her friends and their families, and wish you could help in some way. I would also tell her that during times like these, it is even more important than ever to cling tightly to Jesus’ hand…these types of tragedies can make or break a person’s faith, so encourage her to pray, giving God her sadness, anger, and questions regarding what’s happened.
In the Diary of St. Faustina, Jesus said:
“…there is no way to heaven except the way of the cross. I followed it first. You must learn that it is the shortest way… I am giving you a share in those sufferings because of My special love for you and in view of the high degree of holiness I am intending for you in Heaven. A suffering soul is closer to My heart.”
This is obviously a tough concept for anyone to grasp, but perhaps it will help her a bit…if not now, maybe later.
Here’s an excerpt from the Conversation of the Merciful God with a Despairing Soul (also from St. Faustina’s Diary):
Jesus: What joy fills My Heart when you return to Me. Because you are weak, I take you in My arms and carry you to the home of My Father…
…Tell Me all, My child, hide nothing from Me because My loving Heart, the Heart of Your Best Friend is listening to you."
I would encourage her to also pray for her friends. It sounds like you love her very much, so maybe you can pray with her and/or spend some time with her (go to lunch, whatever) to try to get her mind off things.
Offering a prayer now for you and your niece and her friends and their families.

God bless. :hug1: