Proficiency of a grade 8 piano player?

Well it depends on how long the piano player in question has been playing, and their skill at it. Everybody is different. For example, I am in 8th grade and so is my gf. She has played piano for 8 years and I have for 1. Obviously she is much better. However, I have a friend who has played for 6 years and is better than my gf. Try maybe testing your friend to see how well they play.