The woman, Sarah Horn, even harmonized with Chenoweth in the song which blows Chenoweth away.
If you look at the YouTube page for this video you will see Horn in the comments addressing if she was a plant or not "Definitely not a plant; I promise! I was expecting a plant too which is why when Kristin asked me to come onstage, there was a bit of a delay on my end. Lol!"
I don't believe anyone who uses LOL in their defense. As a rule.