Couple of quick thoughts on this:
The chances of you actually getting sued over this are 0. Sure she might threaten it, she might even talk to a lawyer for about 30 seconds before getting laughed off the phone. But she would have to be completely deranged to actually follow through. She would end up paying more in lawyers and court filing fees than the laptop is worth. I know in our business lawsuits tend to be a big scary thing but the reality is the chances of getting sued are extremely minimal. Even on jobs that are a couple thousand bucks in value you wouldn't likely get sued over something like putting a hole in a wall or a screw through a desk. You would have to do something substantially bad (like cause an entire office to be unusable for days thereby preventing revenue, or sell an expensive disaster recovery product that falls flat on it's face when a disaster occurs) to even sniff a lawsuit.
That said, this is also why we have and pay for insurance! If, for whatever reason, this person did push a lawsuit, just turn it over to your general liability insurance company. Why? Because they have an army of lawyers and personnel to figure this out for you. That's why you're paying for it! Let them take care of the crap. It'll likely be cheaper than paying your own lawyer to try and defend you. Granted, in some cases, that won't be an option but in most cases you'll run up against contacting your insurance company should be your first step. And no, they're not going to raise your insurance premium just because you called them so don't hesitate.
Finally, as others have said, if you don't have one already get yourself a sign-off sheet. It doesn't have to be lengthy or complex or full of legalese but it should state that "**** happens" occasionally during the course of diagnosis/repair and that they agree not to hold you responsible for it. Get it lawyer-reviewed if you'd like to ensure it's bullet-proof but just have something.
And just because we're talking about legal stuff, note that I'm not a lawyer and none of this should be taken as legal counsel in any way. If you need legal advise please consult your own attorney. If you don't have an attorney, contact your local BAR association which can assist in helping you find one.