Yaar, come what may, even if the mother taught the daughter nothing, the daughter was mature and above 18. There is no excuse, no reason that will save the daughter from hitting her mother. There are numerous islamic examples that can be quoted in this regard regarding children behaviour with parents but im not a religious person nor a mullah type to start quoting ayats and hadiths also.
But i do know this, you just cannot hit your mother and father. It carries a very big penalty in this life and after life.
Girl wanting to marry someone not approved by mother is another issue and im not discussing that. Im disgusted that under any circumstances, children cannot hit their parents.
I agree there is no justification for her action ............. but after all if this is a cycle of getting punished in this life for one's own wrong doings then I think nature has done justice to her mom?
Such cases are complex, outcome is not appreciated but the actions are driven based on a number of factors including what a child has gone through in his own home while growing, if that child has seen domestic violence, buk buk chik chik then obviously she would fall prey to any looking for all that missing affection and love. Insecure child when becomes powerful may react the way this girl reacted.