I find it rather difficult for me to respond to your request, as I detest addressing hypothetical scenarios in general principle, because they always fail, at some point. But also, that you request that I limit my response according to your parameters is also difficult. I must say that it seems to loosely follow a construct that has sometimes been used in law enforcement for use in (hostile) interrogations, lol.
I think that, for various reasons, this scenario would be unlikely to happen in my life… so I had to imagine that some random friend that I do know would take the place of “Donna”.
To answer your first question of what would go through my… lol, you don’t want to go there… suffice it to say that my mind would go into over-drive. Yes, I would feel betrayed… in large measure… but I’d also consider that “Donna” is presenting as vulnerable and taking a chance.
As to whether or not “Donna” is delusional… yes, that still applies. If “Donna” had, instead, stated that they were Napoleon Bonaparte, it would be similar to stating that they were of a sex other than that they were born to. The disparity is the same, imho.
As for pronouns, I think that I would be conflicted in regard to that. After all, I had previously been using feminine pronouns for the last four years… confusion, I think, would be par for the course and as I said earlier, I like to stick closely to truth. Perhaps you’ve already noticed the pronouns that I have elected to use so far? Also, I’d feel the same confusion in addressing my friend as “Donna”.
“Donna” is the one who just changed our relationship, and I would also feel the change. A friend has just revealed that they’ve been deceiving me for quite some time. While the deception is somewhat understandable, it is still deception. I would also tell “Donna” that although I was told “in confidence”, I’m essentially being told that they intend to maintain the deception with the rest of the congregation… I’d need time before I could commit to that… and that I see it as another deception for her/him to include me in such an enormous revelation of duplicity that has direct impact on the congregation with the opening caveat of “can I speak to you in confidence”… that’s entrapment, friends shouldn’t do that, imho.