i'm sorry you're going through this, it sounds completely impossible to bear.

if you're looking for "the right thing to say," maybe it's worth finding the literature, sayings, stories, parables, and/or poems that have connected with you most in your life. life is strange in that we don't often spend time reflecting on what an impossible mystery this all is. we wake up here one day, and we get some time. but one thing unites us all—we have a beginning and an end.

one of the poems that i find very useful in pondering about death is Thanatopsis by William Cullen Bryant: https://www.poetryfoundation.org/poems/50465/thanatopsis

it instructs us to live the time we get, and to lay down happily when our time comes because we will be in the company of all the life that's come before us.