Wow, this woman actually called you just to tell you that? How incredibly rude. Good for you for keeping your wits about you and actually manipulating her into being helpful! I would have been too flabbergasted.

Since this is a split classroom already, the grade skip wouldn't involve any physical move at all - just a change in status, visible in the work she'd get and which group she'd go to whenever there are pullouts or extra periods (one day a week the first graders stay for an extra period, one day the second graders).
Right now, the 24 second graders treat the five first graders a bit like pets, enjoying their own superior status while being helpful. DD is tiny even for a six year old, blond and cute, and as a rule hiding her completely outsize personality behind a quietly observant and affectionate demeanour, so, pet status right now works well for her. I worried that sudden elevation to second grader status might turn her from pet to threat in their eyes, kWIM?
The teacher has announced from the start that, considering that she's got first graders who read better than second graders, reading will be grouped strictly by ability, period, so that is not a concern. What I think I might ask for is giving her second grade math work right away and have her take part in math instruction for the second graders, pull outs or extra periods or whAtever. (Science, specials etc. are combined anyway). However, with writing and spelling she is at first grade level, so I imagine she could stay with the first graders for that one, and work on a cursive workbook sat home on her own (which is what the second graders have been working of since fall).
So this could be considered single subject acceleration and she could nominally remain a first grader until the end of the year, when things will be shaken up anyway, officially moving on into third grade, and everyone's had time to get used to things.
Sound feasible? Or paranoid?