For the medical expense deduction only, you can deduct medical expenses for a child who is not your dependent due to a custody issue but otherwise qualifies. Since the HSA benefit follows the deduction rules, you can use your HSA to pay for the expenses of your child.
It doesn't matter whether you pay her, or you pay the doctor. But, you will want copies of the bills from her to support what you are withdrawing from the account. You will also need copies of the insurance statements because there are often insurance adjustments to the amount you pay. For example, a doctor might charge $200 for an office visit on paper, then when you get the insurance statement later, it says that the contractually allowed charge is $150. You might pay all or part of that $150 depending on your deductible, but you don't pay the $200. So get the insurance statements as well as copies of the doctor bills.
