So - please, any suggestions, patterns, ways to alter simple online patterns for this event - fabric suggestions, etc?? I love to make stuff for her, but I am afraid that this one may be a little out of my comfort zone.
Satin will make any pattern look fancy. Look for good quality satin, or crepe back satin, which is really nice to work with, and is often thick so it's warmer than some fabrics. I know i got great stuff for my bridesmaids' dresses that was on $8/m (and I know growing up that's all we ever paid for it for formals).
And don't worry unless you do a tonne of gathers (or overlays) this is no more difficult than sewing any other dress.
Also to further address the weather concern a slightly fancy cardigan or bolero (even just a nice white one) will keep her nice and warm without ruining the look of the dress (my bridesmaids wore white furry sweaters with their dresses to stay warm in the balmy Canadian winter).