Should my son have to carry health insurance on his daughter?

My son has a two year old, whom we shall call Adrionna. Her mother, whom we shall call Voldemort, has her covered under state health insurance. However, Adrionna is also covered by my son’s health insurance through his work, which costs him an additional $30 a month.

Shouldn’t his daughter be covered by the state insurance no matter who she is with?

Or, conversely, covered by my son’s insurance no matter who she is with, therefore having no need for the state insurance coverage?

The state insurance is, of course, far better coverage.