The 23-year-old medical student showed off her long legs in a pair of denim shorts, teamed with a loose-fitting white shirt and simple sandals.
Emily kept her trademark long hair loose and her make-up natural.
Peter was performing at the Music On The Hill event at Kings Hill, West Malling, Kent; a three-day celebration with all profits going to charity.
He joined others including McFly, Stooshe, Jools Holland, Joe McElderry and Gareth Gates at the event, which was held to raise funds for Demelza Hospice Care for Children, Teens Unite Fighting Cancer and The Children's Development Trust.
Peter, who recently launched his own online channel, clearly enjoyed his performance as he tweeted afterwards: "Wow what a crowd :) thanks so much."