Why not rappelle-le-lui?
It would be correct too.
Then why isn't 'en' used in the first part of the sentence: Remind her OF IT?
Because you don't use "de" with "se rappeller", but you do need it with "se souvenir".
'en' translates 'of it'. If someone were to offer you a piece of cake they may say 'Tu en veux?' 'Do you want some?'
rappelle le lui was marked wrong