She may well have been overwhelmed about singing with Donny. I think she has publicly stated that she had a huge crush on him as a young girl. (He was her generation's heartthrob, I think--he was born a few years before her.) Additionally, she's not used to performing live before a huge audience even now; her late career start means that she lacks the years of experience that someone like Donny has, or Elaine Page. What impressed me is that as nervous as she clearly was, her voice remained true and supple. She just wavered on a note or two, but otherwise was on pitch. What an instrument she has! There are people with huge careers who have a lot less of a natural gift than she has. She's clearly emotionally fragile. I hope she's able to sustain her career, because her voice has so much to give listeners.
The other thing that impressed me hugely was how Donny "sheltered" her while she sang. He clearly was trying to calm her, in the most easy, natural way possible. This is definitely one of nature's gentlemen. Considering that he's been a star since he was about five, he seems incredibly free of ego and self-centeredness.