MIL demanding money back after being told not to pay..

So my stepson had a morning dentist appointment last week while we were both at work. It was an emergency appt for a broken tooth and they didn’t have any later appts. MIL (his grandma) agreed to take him to the dentist appt. My husband told her (and reminded her twice) to not pay, that he would pay when the bill came in the mail after. We’ve done this before and the dentist office is fine with it.

Well guess what..she didn’t listen and paid the $45 copay. And ever since the day after the appt she’s been hounding my husband constantly to pay her back. We’re extremely short on cash until next week due to some unexpected bills and can’t pay her back until then. He’ll respond to her with “I told you not to pay” and she doesn’t care at all. Like it’s not his fault you decided to pay when you weren’t supposed to..🙄