her. Or him.

Like JM I can imagine a number of non-privileged ways this Terrance could have non-privileged info he can be compelled to divulge as to when the relationship began. (I for example would have had non-privileged information about my superiors' dating habits, in my first law office back in the '80s, just based on hanging out in the same bars after work, even if later one or more of them had hired me in their divorce cases. Which is why they wouldn't have, nor would I have accepted the cases!)

Whether it dooms the prosecution or not is another question. This doesn't, after all, change one factual element re: the alleged crime.

However, her replacement (if she is replaced) might well disagree as to whether a) the case can be proven, or whether b) the office should expend the funds necessary to do so... even if he/she feels the case can be proven. There's a cost-benefit decision prosecutors have to make in order to satisfy their duty to the public, which is why we have concepts like, well, plea-bargaining and prosecutorial discretion.