Number one rule of colour correction: Always choose your removal method to match the product used.
Unfortunately, as Osmo colour psycho is a direct dye, applying a bleach based mix has caused the colour molecules to permanently stain the outer cuticle layer. Additionally, vivid direct dye purples are usually applied to hair that’s around a level 9 or 10 so if it has been bleached before the purple was applied, it’s already going to be fairly porous and therefore, more likely to stain.
Sadly, I don’t think a direct dye remover is likely to be very effective at this stage.
I think that realistically, your only options are to colour over it and neutralise it or cut it out.
How green are we talking?
If it’s a very pale minty green, you might get away with applying a normal demi tint over it. I think you probably ought to reduce your clients expectations at this stage if she’s looking for a pale ashy blonde. A neutral level 8 might cover it ok. I wouldn’t go any lighter.
In the meantime, purchase a direct dye remover and a colour reducer for oxidised colour to keep as your essential supplies and read up on the best methods and products for removing different types of colour so that you are properly prepared for dealing with this type of situation next time.
Let us know how you get on.