I agree, she does not have any say in your life. If she wants to be an idiot over this, then there is nothing you can do to change that. Just dont stoop to her level, do the best you can by her son, and at least you can hold your head up high huh?! In time, she may even relax over the whole situation, and although you guys may never be buddies, she may come to a more peaceful truce.

Just a little thought, I would not use the words of wanting to be a mother figure near her, or her son. Best to describe yourself as being the son's friend. She won't take kindly to the word mother, esp as she's being so difficult.

Good luck and I hope things go well.