Lewis,
I think you need to be careful here. Clearly you are dealing with a problem customer. Ordinarily in the situation you have described the customer would ring you saying they've spilt yoghurt on the suite and come you come and fix it. If the stain doesn't come out they would see it as their fault, not yours. Your customer seems to be blaming you for what she did, which is ridiculous. Having been out to attempt to remove it for them and having failed, I'd leave it at that, here's why. If you continue trying to clean it this type of person could try to blame the permanence of the stain on your attempts to remove it, claiming you've used harsh chemicals etc, they've already blamed you for something they did so they could take their illogical stance a step further. As it stands, you cleaned the suite, which they were happy with, then spilt yoghurt on it afterwards, you've been out to try to remove it. You've done everything you need to do, so leave it at that.
Simon