As already mentioned, there is no harm in purchasing the colour in for her, put her name on it and it aside for use on her.
However, this issue for me would be... is this the only colour she intends to use otherwise she may expect you to do it for her other colour choices? As an untrained and certified nail therapist, she is not entitled to purchase her own bottle of Shellac and yes, she would then know what you paid for it. I also do not know why she wants her own colour but not base and top coat.
I would simply but politely say sorry, but not possible. You are a trained therapist, this is what you stock and whilst you can order a colour for her, it will not be for use on her only.