Sounds like a potentially very serious mistake was made, and because of her size someone didn't stop to think about her breed, and assumed she was old enough. I think she was just incredibly lucky to have actually survived the op, especially a spay instead of a castrate.
My only concern now though would be as to if there are likely to be any behavioural/temperament problems because of this, as I know dogs temperaments can be messed up if time of spaying is incorrect, and when Pippa was first booked in, she then had a 'Phantom Pregnancy' and so it had to be cancelled, until she was through it. I'm just wondering what the implications may be for bunnies neutered when not sexually developed??