Herman was not a kind person, he had done many evil deeds and he hated Jerome for getting all of Elizabeth's love.
He was jealous of Jerome and even detested him. Even so, Herman was not a foolish man. He clearly knew the only obstacle between him and Elizabeth was the woman herself.
She was simply too powerful.
However, he could easily have Elizabeth to himself now as she lost all her memories after being brainwashed. Hence, he had teamed up with Jerome from the very beginning.
Dona was specially arranged to meet him after the discussion with Jerome.
Jerome wanted to live happily ever after with Connie but Elizabeth...