Metformin impairs the mitochondria's ability to use glucose as a fuel, which appears to have two effects: it reduces glucose use in the body, but it also reduces glucose production in the liver—which is the major cause of hyperglycemia (high blood sugar) in type II diabetes.
"...Sub-analysis of non-alcoholic fatty steatohepatitis showed that metformin failed to improve any pooled outcome. Adverse events were poorly reported. Current information indicates that metformin improves liver function, HOMA-IR and BMI to some extent, but not histological response in NAFLD patients...."
I suppose this is because the mitochodria still produces oxidative damage in the context of excess linoleic acid consumption, even if they're using fat instead of glucose as fuel.
More study required!