I thought she agreed to go on a date, not to become girlfriend... atleast yet.
Probably due to awkward translation... i would guess that it was 憑き合って (tsukiatee), which can be used as both "accompany me" (which not only one time, but also can be without romantic sense whatsoever)... and as "please go out with me" (as actually dating), as well.
So i kind of understand why mistake could have been made.