Like all smuggling, it's a risk to be taken or not. How lucky is your buddy, in general..? Folks travelling out and back with stuff (not just instruments; cameras, expensive watches etc...) would be wise to have their original purchase papers with them, to show Customs that all taxes and duties have already been paid. If the final destination is not a, EU country, VAT would not be payable, but import duty of the final country would be. The documents required for 'sending on' in that way are rather complex, and probably beyond the scope of yourself or your friend. I'd have a real think about the whole deal if I were you. The odds against are stacking up.
Just sayin'; hope this helps.