see pub 969.
“A child of parents that are di-
vorced, separated, or living apart for the last 6 months of the calendar year is treated as the de-
pendent of both parents whether or not the custodial pa-
rent releases the claim to the child’s exemption.
Clearly the ex’s HSA can be used for the child’s expenses. What I don’t understand is why you are involved in the payment of the expenses. The HSA is the ex’s so they should pay the expenses and those expenses would be reported on their return.
