I assume that the donor did not ask that their identity be kept a secret from the founder.  I also assume that the founder is not a current corporate officer who would have a legal right to know the donor’s name in order to do their job.  Therefore, it must be a conscious decision by board leadership (or perhaps the whole board) to try to keep it a secret from the founder.   

The primary reason that I see to keep the founder in the dark is to alienate the founder from the organization.  If you want the founder to go from upset to absent, or want to curb the founder’s enthusiasm for helping the organization, this is a real good way to do it.  Let the founder try to get the information from friends on the board, who say they don’t trust the founder with the information.  That will definitely affect the founder’s desire to help.  And it will put your directors in the very uncomfortable position of having to choose between information for a helpful old friend and your arbitrary confidentiality rule. 

Even if the donor asked to keep the identity secret from the founder, I would recommend that you ask for permission to reveal the information on a confidential basis.  Founders can bring perspective, contacts and dollars to an organization.  They can also undercut it by bad-mouthing a group from which they have been excluded.  It is hard to maintain a good and productive relationship with a founder who is active but not a central figure in the organization’s current activities.  But if you think that is worth aspiring to, you will keep the founder informed and involved, not in the dark and alienated.