I have used NSI products for decades now and I am confident it is not the products but I would suggest to stop using acetone to clean your brushes as it will contaminate your brushes and product long term and contribute to a lesser life for your brushes. You should only ever clean your brushes with clean monomer
What does this client do for a living?
It is obvious it is the client who is causing this problem but she doesn't realize it.
Does she wear plastic gloves for work or a lot around home, not the rubber ones but the thinner ones that you through out after every use?
I know from first hand experience and with some of my clients, if we wear the plastic and coloured gloves, our nails can turn a bit yellow/brown.
There are many issues as to why this could be happening to your client but I would suggest you don't go blaming yourself because if clients previous and after her are all ok, then clearly it's something she is doing, be that plastic gloves to trying a lotion in a shop that has reacted (or has tanning ingredients in the lotion) and she has forgotten she has done this, there are many variables as to what she has done to her nails to cause this, I would gently, nicely but sternly let her know that it's her and not you so that she knows she can't put the blame on you or the hard word for a refund.
Is all you can do is keep playing detective and think outside the box as to what could be causing it but you might never know