She’s got some family member following her around dropping the identical balls in the cups.
I pulled this same crap as a kid. We played in following foursomes. I would have a friend of mine in the group ahead of us drop a ball like mine in the cup as they replaced the pin. I would shank one in the trees (without even trying!) and someone else always found my ball in the hole while I was picking up my real ball in the trees.
I did it as a joke.
Find out who was in the foursome ahead (probably her “partner”) and you will know who was dropping the ball.