The first thing needed to be done is to carefully read any signed agreement that you and the other person signed concerning the project and look to see if there was anything mentioned about reimbursement of expenses in that such document would control the obligations owed between the two parties on the project.If nothing is stated and expenses were incurred where the other party is no longer working on the project, you should quickly reimburse him or her for all reasonable out of pocket expenses incurred. From a business perspective it is better to pay the expenses and move on especially since the work done was "gratis".