Father applying for Medicaid soon. He currently pays private health insurance premium that covers him AND mom. Will mom lose her insurance?

Do you mean Medicaid for a NH or health insurance. If health insurance, why isn't Mom included.

Call the insurance company and find out how this works and what needs to be done to keep her insured.

Just to clarify.
Did you perhaps mean that your dad is about to apply for Medicare?
How old are your parents?

They can continue the healthcare if they want. There is somekind of adjustment made if he pays out of pocket. But Dad really won't need it. If he had Medicare Medicaid becomes his secondary. He gets dental and eye too. The premium will be lower if its just Mom. May want to check and see if she would qualify for Medicaid health insurance at the Community spouse. She should be able to keep the insurance just drop Dad. Call and ask the insurance carrier.