It sounds as though he would have a tough time overcoming the written evidence that it was a gift, but that doesn’t mean that he might not try.  In this case, that type of claim is likely to be based on how he remembers discussions with the charity at the time of the event.

Assuming the charity thought it was a gift, what other corroborating evidence does the charity have?  Does it have a copy of a gift acknowledgement? Was there any other correspondence? Does the person who received the gift have a clear recollection of the events? Was it part of a specific campaign? Did the charity list the donor in its annual report?  Did it thank him at a public event?  Did it send him an annual report from which he could see that there were no loans outstanding? Does the board normally authorize any borrowing, and are there any minutes of a board meeting approving this loan? Did the charity ever pay any interest on the loan?

If the charity stands by its position and provides good evidence to the donor, most donors will not be tempted to sue.