What does Pausch's wife whisper to him on stage?

I believe you're referring to the penultimate chapter in the book where Randy focuses on the effects of cancer on a patient's family. He wants everyone to know just how much he loves and appreciates his wife, Jai. Randy would much rather have cancer than be hit by a bus, because at least with terminal cancer he'll have the opportunity to prepare his family for the inevitable end. He'll miss his family terribly, and in particular he'll miss seeing his kids grow into teenagers. But he's sure that Jai will be just fine. Yes, she'll make mistakes, but that's all part of being a parent. And Randy isn't worried because he knows that Jai will be surrounded with friends and relatives who'll give her all the love and support she needs.
Randy missed celebrating the exact day of Jai's birthday, but now he leads the audience in a heartfelt rendition of "Happy Birthday." An emotional Jai walks onto the stage, embraces her husband and whispers "Please don't die." Although Randy will eventually pass on, that special moment of love and devotion will endure for all eternity.