The green colour is the result of a bacterial infection ... It is not fungal.
You must completely remove all of the product or the bacteria will not die.
Oxygen from the air will instantly kill the bacteria & usually a green stain will remain depending on the severity.. A light stain may buff off easily..but if it doesn't then you will have to allow it to grow off the nail as you don't want to over file/buff & end up thinning the nail out.
You will need to thoroughly cleanse the nail with a product such as scrubfresh that has pathogen fighters in it.
You will also need to discard any files that have been used on this lady.
It sounds like the most likely cause of this happening is that the nails have lifted... Any lifting that is left or develops will allow water & dirt & such like to get between the nail plate & the product... & then the bacteria starts to grow & multiply.
My first course of action would be to get those nails soaked off completely & start again x