Ignore the 6 months. Those commas should be read as OR statements.
A child of parents that are [divorced1] OR [separated2] OR [living apart for the last 6 months of the calendar year3] is treated...
You meet the first OR statement, you can ignore the others.
A person who owns an HSA may use the funds to pay for qualifying medical expenses for themself, their spouse, and their dependents, including children who could be their dependents except for divorce.
If they don't want to pay, that is something you have to work out with them, possibly through an attorney. They aren't required by the tax law to use their HSA for anything, including their kids. But it would not be taxable to them if they did use the funds in that manner, provided that the children meet the other tests to qualify as dependents (under age 19, or under age 24 and a full time student, does not support themselves, etc.)
