Reality? Some people can wait for a lifetime for someone to come back. Some people can't. She was completely alone and Fernand played on her loneliness. I still feel sorry for her.
Mercedes jumped in bed with the first General that came along and on top of that didn't see to it that Edmond's father was fed and taken care of. She got off easy, besides she got old and the Count now needed a younger prettier girl. That's life.