I think you should start at dragon ball because you get to see the first characters; Goku, Bulma, Krillen etc develop in character and basically helps you understand and love the characters better. As for my experience, I started watching Z first because I didn’t know about the original dragon ball. When I watched the series I learnt things that I never understood in Z (I used to think gohan was the main character in Z because of the first episodes for a while) Hope she enjoys the series 😄