The company may be flexible, but if she still has to work the same number of hours, it sounds as though you were asking her to arrive half an hour earlier for work that morning. I don't blame her for not wanting to, as it comes out of her sleep. Especially since she is likely racing around to get ready for vacation. Sometimes the getting ready for vacation is pretty stressful. Pack, clean your place so it's not a sty when you get back, take animals (if any) to boarders, or leave instructions for house-sitters. I'd be loathe to get to work earlier than usual too.
I'd say it's fine to suggest an earlier time, but if she doesn't want to change her work shift, either take the invitation as is or decline.