If someone has two health insurance coverage, how does coordination of benefits work?

My friend was in an auto accident, and the health insurance from the auto insurer is her primary health insurance for the accident. She also has regular health insurance from another company. If the auto insurer pays 80% of her auto accident medical costs, and her secondary insurance normally pays 70% of her medical costs, then how would the secondary insurance treat a $1000 bill, for example? The auto insurer pays $800 of the $1000 bill (80%), but how much would the secondary health insurer pay? SRC50 Auto insurance covers medical costs from an accident when auto insurance is selected as the primary health insurance for an accident. She therefore has two health insurance coverages in terms of her auto injuries only.