First let me say that I’m very sorry for how this woman has treated you. You don’t deserve that. I understand this is your best friends mother, but all you can do is delete her from your life, from facebook. It may be hard to do since she is someone that has been in your life all these years, but she is toxic to you. If you weren’t friends with her, you would never know that she was still liking and engaging with your ex. Put an end to it now. She is not your friend.