I think what I would recommend in this situation is to contact the mother privately, phone call, personal email and say that you are getting ready to tally expenses from the bachelorette weekend and to contact all the bridesmaids and let them know what each person owes. At that time you can simply ask her if she still planned on graciously accepting the fees for the hotel, and if so, you would make sure not to include that in the costs that the bridesmaids need to take care of.
Whether she decides to stay true to her offer or not, you need to tally all expenses and divide it between all the girls. It is an unwritten rule that when accepting to be a wedding party member you are taking on the role of helping to pay for bridal showers, bachelorette parties, bridesmaids gowns, and accessories, and a wedding gift, in addition to shower gifts, party gifts, etc.
It is best to set a plan into action before all of the parties stating what each attendant is comfortable paying per person, but it does sound like you are past that now and will have to deal with what was already spent.
I hope this helps somewhat. Best of luck!