Can my son be covered under his father's girlfriends health insurance?

I was just notified by my sons father that his current girlfriend would be covering my son under her health insurance policy through her workplace. It was stipulated through the courts that he was to supply coverage. I don't believe that she should even be able to cover my son, I am the custodial parent so its not like she can claim that he lives with them.
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It's possible, but not probable. Domestic partnership rules are different company to company so the getting a letter from the HR department acknowledging that they understand that it's her domestic partner's child is a good idea. You don't want a large claim to find out that it's going to be denied because they failed to mention that on the application.
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