Can employer refuse to insure an employee who’s covered elsewhere?

I’ve attempted to find the answer online but am struggling.

Long story short: the company I work for recently switched insurance plans. My coworker was told that because he is covered by his wife’s insurance (wife works in completely different industry)he does not qualify for coverage under our companies insurance.

Located in Washington, and he is a full time employee who was dually insured prior to this switch.

Is this even legal? He’s missing out on a huge portion of our benefits package, and offered no compensation in leu of the insurance package.

Thank you!