I think it is fine to tell your deceased friend's relatives that you wish them well but for personal reasons you are not going to commincate with them on Facebook any longer. It isn't necessary to provide an explanation to them as to why. I do not think it is necessary or appropriate for you to recommend they now connect with 'Susan'. Better to leave how they spend their time on Facebook up to them.

By the way, I am sorry for the loss of your friend. Although you had your ups and downs, it sounds like you had some nice times with her and she had significnt meaning in your life.