(Part A doesn't cover ER visits, or any other out patient services. That's Part B. Check your husband's Medicare card. It'll list what he has.)
Make sure the hospital has all the info for billing both your primary insurance and Medicare.
Typically Medicare pays 80% of what they term "allowable cost". If your primary insurance covers less than that, they will pick up the difference, up to that 80% of allowable cost limit.
One request that's always worth making is to ask the hospital to accept "assignment". That means they agree to accept insurance/Medicare payments as payment in full.
Another is to ask for assistance from a hospital patient services representative or ombudsman and a discharge coordinator.
Note that physician services may be billed separate from hospital inpatient costs, so the same steps apply to the physician billing. And ask them to accept "assignment" as well.
Good luck. Hope your husband is fine.
On line info is available at
http://www.medicare.gov or at 800-633-4227