Grading Singers for Final Performance

So, as well as a singer, I am an accomplished pianist/accompanist. I have been playing for about 10 years now and have worked aside several levels of singers and instrumentalists. With that said, I have the privilege of playing for a friend of mine during her final exam. She sang 'My New Philosophy' from 'You're a Good Man, Charlie Brown'. The song is originally a duet, but she chose to perform it as a solo. Last minute, someone else in the class decides that he wants to perform it with her. All parties were in agreement, including myself! So we set up a rehearsal time to go over the music, in which I was present, 'Sally Brown' was present, but nooooo 'Schroeder'. This made me a bit uneasy. He didn't only miss our rehearsal, but then went on to use this song as HIS final and not as a supplement. So it's our time to perform, and we head up on stage. So this is my FIRST time running this song with him, and it's the actual performance. How fun and unprofessional is that?!?!?! So we get up there, and everything is going smooth, then all of a sudden, 'Schroeder' jumps about 3 pages ahead and cuts out more than half the song, so 'Sally Brown' does an amazing job following him, but at the same time, I am kind of lost. I eventually find out where we were, and before you know it, the song is over. And do you know that 'Schroeder' got an 'A' on his performance?? Not only did he NOT song, but he only spoke about 15 words. Talk about favoritism!!!