Many years ago, when I was still messing around with consumers (a no-win proposition), I had to drop customers less problematic than what you're describing. With the complexities of the Web, many browsers, and social networking, things have only gotten worse as to what the consumer might consider you responsible for.
Why are you even involved at all, when you never supplied that computer to her. Your only responsibility is to your original customer, not the person your customer supplied this computer to. Your original customer had no issues with you over this computer, right?
If this second owner had come to you in the first place as a (primary) customer, you would have had the chance to consult with her, hear her out, and possibly reject her as "not suitable" for you to do business with. She cannot indirectly become your "forced" customer.
Hartmut Sager