If you haven't found this website already, you might enjoy taking a look around. Here's an entry for metformin Hcl. From reading through that, there's no testing they've done on people with hepatic impairment, but they have done tests on renal impaired folks, because one of the serious side effects of metformin Hcl is lactic acidosis, which is pretty darn fatal (50% die).

It seems while metformin does do stuff to and for the liver, it doesn't actually go through it. But the kidneys have to process the darn stuff, and that can put a strain on them.

So either she's stupid and confused liver for kidneys, doesn't understand how this stuff works, or is actually resistant to it after a while. But there's no detoxing of the liver that's going on there.