What's wrong with onde fica ela?
Take a look at this thread: https://www.duolingo.com/comment/2233756
"Estar - indicates where the object is at this moment
Ficar - indicates where the object belongs to/is used to be"
Hope this helps!
Because "Fica" here would sound like "stays" and that would means that she is always there like something with no skills to leave a place
Why not just onde esta?
Why I can't say "Cadê ela?"
That's right too, and informal...