Other than statutorily protected information, such as personal health information or certain academic records, that a consultant is legally permitted to obtain in the course of the consulting work, I am not sure that you can count on much of the information that you give the consultant being confidential unless you so specify in the contract.  It is not at all unusual to provide that a consultant cannot disclose, without your prior approval, any information obtained in the course of the contract work.  Obviously you want to exclude any information that is already in the public domain or information that has to be disclosed pursuant to law (although perhaps only after prior notice to you so that you could contest the legal requirement), and you may want to protect only certain information, such as donor records or trade secrets.  But if you have concerns about the information, you want to have a clear understanding in writing before you start the work.

You might have a better legal case if the consultant goes rogue and misrepresents a relationship with you in order to get what you consider confidential information from someone else.  But you don’t want to have to rely on litigation to protect your information or the consultant’s use of it.  Good due diligence on the ethics of a potential consultant is probably better protection than the possibility of a lawsuit.