If you have talked about the performance and have determined that your student is anxious about playing, we can then try to find out what the anxiety is about. Some students are afraid of performance because they haven't performed before, or they haven't performed in a formal setting before.
To help students with this particular anxiety I will sometimes take them to the recital space and have them scope it out. Feeling comfortable with where they are playing can make them feel a whole lot better.
We often do a mock recital in lessons where I call up the student and have them bow and then play their piece and down again and sit down - we practice their performances. It feels silly when we are doing it, but I've noticed with my students who are seriously nervous about the recital this can help.
Try doing this at home - playing for more people and practicing the recital protocol can help anxious students feel more relaxed when the time comes for them to play.