Someone I have known well for many years, is selfish and attention seeking, she delights in other people's misfortune and often bites the hand that feeds her. She is not a good friend to the people she classes as her friends.
She is not happy. So much has gone wrong for her, but she does not learn from her experiences and keeps repeating the same mistakes.
She is machiavellian and loves to plot and plan the downfall of others.
Normally this back fires on her.
Would it do any good to say "What goes around comes around" to her ?
"Please don't come to me for sympathy. I know you are not happy, but you are selfish and attention seeking. You don't learn from your mistakes and just keep repeating them. You delight in other people's misfortune and turn on people who try to help you. You are not a good friend to the people you class as your friends. It is foul of you to enjoy plotting to hurt other people."
I mean, if this is the person that she is - she sounds vile and a dose of reality might be well overdue.
I would just try and have as little dealings with this person as possible. Apart from anything, sharing this home truth with her would probably just make you the next target of her little games, out of spite.
YAB a bit U. Mainly because what goes around rarely does come around. People who treat others like rarely get their come uppance. IME the worse they are the chances of something bad happening to them in return reduces.