How much acetaminophen will cause liver failure?

My friend has been taking norco 10/325 for the past 6-8 weeks. She recently confided that she is taking about 2-3 pills at a time, 4-5 times a day. That's 3900 mg of acetaminophen a day, plus she told me that she supplements with regular pain reliever. I asked her what it is, she said acetaminophen. She thought they were 200 mg tablets, but it turns out they are 500 mg tablets. She takes 2 of those at least 3 times a day. So now she's ingesting somewhere around 6000 mg per day, if not more. Is she slowly killing herself? She said she has no symptoms, but could it still be hurting her? Does anyone know what dosage in what period of time can be fatal? Thanks