Kvod harabonim Shlita: I am in the wedding gown rental business. I rent both used and new gowns. The Kalla who rents knows whether her gown is new or used. The price of rental depends on the appearance, condition and the amount of work involved in personalizing the garment to the fit and taste of the customer. In the case in question the seam which by which the zipper was attached to the dress opened (about 5 inches, apparently the seam was not sewn well) during the Chasuna. Other than the time which was required for emergency repairs the Kallah had full use and enjoyment of the dress. On the advice of a Dayan I refunded the mother of the Kallah $200 for the עגמת נפש. The customer is not satisfied claiming that she is entitled to much more since she paid $2200 for the rental.
It is impossible to answer a question in Dinei Mamonos without hearing both parties present their case.
In this particular case it would depend on various factors, including whether the seam was faulty (as seems to have been the case) or whether the seam split as a result of misuse at the wedding, how long the emergency repairs took and how effective they were.
There may be circumstances in which no refund is required al pi halachah but in which it would nonetheless be advisable to return some of the money as compensation, but how much would be involved is subjective and can only be suggested once both parties have presented their positions.
My advice is that both parties agree to abide by the decision of a particular Rav or Beis Din, who will be given the opportunity to hear both points of view.