Maybe. Maybe not. If this is a swim team it probably had to be a swim team style suit, not a justice or
walmart bikini.
Here is my take. The dad is not asking anything from the ex-wife. He is asking his daughter to reimburse him for her mistake. I have to take the dad's side on this one. He knows she gets an allowance and his decision to discipline her is for her to repay him. I think the mom getting involved is undermining the dad's authority to discipline.
Example: If my child broke a vase in my home I may say they have to pay me to replace it as punishment. If my child broke a vase in their father's home and he says they have to pay to replace it as punishment, but I as the mom say too bad, so sad, the child does not have to pay... I am undermining his authority as a parent.
The allowance belongs to the child, not the mom. Both parents are involved here and the child needs to learn she has to follow the discipline of both.
I wouldn't dare say the dad needs to also give a separate allowance unless I knew the full details of the child support agreement, custody arrangements, etc.
Really $40 is not worth all the drama. The child will learn a lesson about not forgetting items. I hope the child does not learn the lesson of how to play mom and dad against each other and have them argue over little things like this.
Once the parent states the punishment, the parent should not have to retract it or else the child learns the wrong lesson. If I was the mom and upset over the situation I would speak with the dad and say I will support him this one time, but in the future if he could speak with me next time, I could save them hassle and money and bring the item to them.