@Emma, I’m pretty sure I will be taking GCSE Japanese, although not via the School. The School is aware of my Japanese learning studies. Actually, I was one of the few Year 9 students who was highly suggested to take the English Laureate course (or whatever it’s called) so I have a better chance of getting into college and I was required to take a language and a humanities but the language requirement has been dropped because of my Japanese studies outside of School.
I meet a Japanese tutor once a week (except during School holidays and the Summer holidays) for one hour and she has Japanese GCSE papers so I will most likely be doing the same as your friends sister. However, apparently one of the requirements for taking the Japanese GCSE is knowing 600 kanji and (I’m not completely sure on this) but my Japanese should be somewhat near a 6th graders level and my Japanese at the moment is a 2nd graders level (going by the Japanese School years) so if I meet the requirements by working hard then I should be taking it. :)