Two things: 1) Which instrument??
2) Have you looked into the Trinity Performance Certificate? I forget the exact name, but it's post G8/pre Diploma, and it might give him something to work towards, possibly fend his mum off the diploma warpath, and, unless he pulls his finger out and works on the deeper listening side of it, he may end up with a fail and be brought up short/forced to do the donkey work to progress...
I ain't a diploma level player just yet

but I do know that most people who get grade 8 with distinction still leave a lot of time/do a lot of maturing/do a lot of practice between G8 and diploma...
Maybe his mum thinks it's grade 9 - would it be worth explaining that most would consider it at least grade 11 or 12??