In summary, trustee of a voluntary organisation wrote to the chair expressing concerns about some aspects of the way the trustees were working, chair responded emphasising that her response was confidential, not to be copied, shared etc. No idea what the response was, I'm neither of them, but I understand the recipient does not think the response should be a secret from other trustees.
Can the chair in fact impose confidentiality when she's answering something written to her by a trustee in her capacity as chair? Should any aspect of her chairing responsibilities or views on the running of the charity or trustees' business be kept secret from other trustees? I could understand it if the response contained information about her personal circumstances, e.g. her health, but that doesn't seem to be the case.