This is a very good topic, though a really sucky situation. Something that do strike me is that this happened in front of your wife. Perhaps she is also trying to reasure your wife (and perhaps even herself!) that she is "normal"? It might be wishfull thinking, but you know for sure she aint 100% brainwashed and a little wishfull thinking is good given the circumstances.
With DF'ing, the standard tactic is to frame the subject so you either discuss personal experience like above, disfellowshipping of pedophiles, or statements like this: "Usually, it is my experience that when someone is disfellowshipped, there is usually more wrong than (something you brought up)."
As you have learned it is surprisingly effective tactics, given how idiotic and (pardon an atheist for saying this) unchristian the DF'ing is.
I have had a similar conversation once. My tactic was to give them that DFing is not automatically wrong all the time, but that I dont like when it is applied to things which are not in line with the bible. I dont think anyone would not take that bait hook line and sinker and ask what acts, preparing to burry one scriptually :-).
At this point, i picked a changed teaching on blood.
The good thing is that "new light" is really the only explanation they have had drilled in, so its bound to come up. But it does not really fix the problem, since by saying "new light" they admit the teaching was changed which just reaffirm what you just said: People are sometimes DF'ed for acts which are not prohibited by the bible.
What i got from here was that any system can make errors, etc. etc. etc. I said the reason it didnt sit well with me was because i felt it was the only explanation the pariseans might have given to jesus: They punished people for healing on the sabbath because that was the best they could read from the torrah, but jesus was still angry at them because he felt they should have used more compassion.
Its not that it really changed anything, but the responses did fell a lot slower near the end of the conversation and was outside the "textbook" than to begin with.