If the dog is elderly, its highly unlikely she is going to ever become dog friendly now, whatever you may try to stop it.
I know nothing whatsoever of your dogs past life or her basic temperament and outlook on life or training, so therefore its almost impossible to give you any advice.
Also if she acts the same on or off the lead.
There are quite a few reasons why dogs can be over aggressive towards other dogs.
eg: Dominance, Fear, lead restriction, lack of socialisation when young, Trauma with another dog when young, lack of training by owner.
GSD owner for 56 years.