Firstly I wouldn't give any guarantee that it will come out. I always say to the custy first, are you going to make a claim on the insurance if it doesn't come out. If they say yes.....I tell them to go that route first.
I've done many nail varnish stains over the years and many do come out with acetone and prochem citrus gel + HWE. Some however will not come out as there can be a bottle of nail varnish spilled and it's just too dense. Also it can dye the wool.....so sometimes there is no chance.
It's the same old story really, you have a dedicated customer and you want to do your best.
I would just say it's a 50/50 chance that it will come out without making any promises and they would have a call out fee.
Mostly if it's a new enquiery, I tend to fob these jobs off, but with it being an existing customer you have to try your best sometimes.