DEAR MISS MANNERS: I find it mockingly insulting when a girl that you previously dated, and asked to an informal dinner, asks what the dress code is.
It puts pressure on the situation to make it more formal than it is, and what if I said formal (as in, I am trying to date her again) -- would she take it the wrong way and get insulted? Or what if she IS interested and I say casual, and she gets insulted that it is not a date?
Either way, I argued that for an informal occasion, the question should not even have been asked.
GENTLE READER: It seems just possible that you have concerns beyond the level of formality of the dinner, but Miss Manners will answer the question you have posed. Assuming that you told the lady that the dinner was to be informal, why would she ask again? She may have a nefarious purpose, as you suspect, or she may simply not have heard you the first time.
You might also consider that the definitions of formal and informal (not to mention the ubiquitous but fuzzy meaning of “casual”) vary enormously depending on the context and even individual interpretation. Is it too much of a stretch for you to consider that your friend is merely trying to figure out what would be appropriate for her to wear?
Whether or not communication was a problem the first time you dated, if you truly intend to begin fresh, the wisest course -- and the most civil -- is to understand text before delving into subtext. Assume her question was an honest one, and answer it.