"He had just a normal health insurance plan."

So, July-Dec, not eligible for HSA if the coverage doesn't qualify. He only gets to put in half. You can make a corrective distribution.

A lot of people think the HSA is part of their coverage, but it is based on their coverage.

It's like having an IRA or Savings account. I can have an HSA account, which I would contribute to only when I was covered by qualifying HDHP coverage, whether that was through an employer, through a Marketplace, or any individual/family coverage from an insurance company or broker, as long as the coverage qualifies. For any times I was not covered under a qualifying plan, I still have an account and use the account for Spending, but no longer qualify to contribute.