I think your advice to the straight cisgendered woman who was aching to out an ex as transgendered was correct; her acquaintance’s right to privacy far outweighs her desire to “save” others from his supposedly shocking gender identity (a transphobic assumption, obviously, that other women will be as unnerved as she was). A recent post on the blog Feministe discusses an aspect of outing trans people that you did not mention: it can be extremely dangerous to the outed person.
Trans people face disproportionate violence and discrimination in the US and elsewhere. The woman who wrote you is clearly concerned about her religious community’s welcoming of a trans man; her outing of him would surely carry more negative consequences for this man than it would for her and may even make him a target of hate crime. The answer to the question posed by your column’s headline (“When to Out a Transgendered Dater?”) is “never.”
Thank you for making the right call in this scenario.
Update: I’ve read some other responses to this column from people who find it very offensive and read Cohen as blaming the trans person for the situation. This is not my interpretation of his column, but I recognize that that might be because I am not sensitive enough to transphobic rhetoric.