Exactly! "her" is a possessive pronoun when "she" is a nominative pronoun which is correct in this context. If you were to say " I spend more than HER father" then it would be correct because I'm referring to something she possess.
For heaven's sakes, which language are we learning here?
I thought it was Portuguese - and everybody is teaching English!
Anyway, Duolingo should accept both "she" and "her"
and we can go on with the lesson.