If you know the woman from class or elsewhere, just treat it as two friends going to dinner.
If you don't know her and she just seemingly randomly contacted you, most likely, someone from the chapter (it could be a recent alumna, as in someone who graduated last year) knows you and knows her and thinks you would get along well. Again, just treat it as two friends going to dinner.
This is how deferred rush is SUPPOSED to work!
Make a friend first, and THEN worry about rushing her.
(ForeverRoses - I got the impression this was at a campus restaurant or snack bar - not the sorority house.)