My friend is a bass player and is trying to find an unaccompanied bass solo piece to play for an AS Level Music performance (standard should be around grade 5 or 6)

Anyone have any suggestions?

Thanks for reading
Well for my next performance I'm doing a bass version of Bach's Toccata in D Minor which is moderately complex, couple pages long, I could probably upload it if you want
When you say unaccompanied do you mean without other people or without any backing whatsoever, e.g. a backing track?
and is he allowed to play a piece with a backing track and play a solo?
She wishes to play it unaccompanied... I personally think she'd be better with a backing track but yeah, unfortunately it's not my call!

So completely solo!
Thanks for reading
What is he doing for the other performances? (and my advice from having done it last year is to aim for grade 6 pieces (or grade 5 pieces with singing) so that he can hit the more difficult mark boundaries for each piece)
A cool thing to do is a two-hand tapping version of Charlie Brown's christmas thene like Stu Hamm has done. Search for it on Youtube, it looks pretty crazy.