Please see the following ruling from Aqa Sistani:

1305. A person whose journey is not haraam, nor is it for a purpose which is haraam,
should shorten his prayers even if he may, during the journey, commit some sin like,
indulging in Gheebat or taking alcohol.

What is the case (with respect to prayers and fasting) of a woman who is travelling to a function out of her hometown where she will not observe hijab, although she might or might not be observing hijab usually?

If the function it self is not haram, then she should shorten her prayers and not fast