It sounds like you are right and this woman is out of bounds. However, I just wanted to put a few thoughts out there for you to consider. My sd was in a much more difficult situation, in that her birth mother basically removed herself from her life when she was 12, and she and her brother were living with dh and I. DH worked alot and went out of town alot. many situations would arise where i was the only person to be her advocate and so I would march to the school or wherever to do that. At first it felt very strange, like it wasn't my place, but dh really wanted me to do that for him/her. Eventually I would naturally respond as her advocate whenever a difficult situation came up. The birth mother (WIL?) would periodically decide she wanted back in their lives until they (the kids and her) would have a problem and she would withdrawl again. I know that in those times that she was trying to participate she probably resented the fact that I was sd's primary advocate.
I know that your situation is very different, as you are clearly a very involved mother, but I thought you might look into the situation to see if there is some reason your son might have felt a need to appeal to her as an advocate, or that she on her own would have felt he needed an advocate. Then you can address whether your issue is just with the technique she used (ie: a scathing letter) or with her assuming the role of advocate at all. I would be clear which thing you are upset about before you attempt to resolve this with her.
I hope this helps some in working this out with her so that all parents birth and step can work together for what is best for him. I know I really wish I had been able to work with my SD's birth mother.