Regardless of any physical disability issues the woman you described sounds very immature to me. And one that likes to involve herself in others drama. Maybe her disability limits her to access to social interaction (which it shouldn't, if she can go to school to harass your son, she could get to other "social outlets" just as easy) and getting over-involved in her daughters life is how she connects with the world...some parents just like to live vicariously (is that the right word?) through their children.
Personally, my son and I are best friends. I am a parent first, best friend second. I provided strict rules and guidance for him as a child, but as he matured we developed a very close bond and friendship. He calls to share any and all types of news and happenings in his daily life, but I don't get involved in drama...but then he is not into drama himself, so there isn't much of that. We tease him about being the "poster child for Murphy's Law", so there is just some unavoidable drama that just comes with daily life. He feels comfortable enough to tell me anything. In fact, I have lost count of the times my response has been "TMI" but we just laugh about it and go on. Our sense of humor levels are very high and we love to laugh with each other....and we are never together that we are not laughing about something.
I was very thankful for the strong bond we have when he joined the military. I valued (and still value) all the time, memories, and life experiences we shared and he knew he could count on me to take care of things for him that needed to be handled while he was deployed.
But I don't fight his battles for him. I don't involve myself in issues he needs to take care of himself. He is very independent and strong and he would not have gotten that way had I stepped in to every little issue he ever had. I did not want my son to become a "mama's boy"
I feel that parents need to develop an appropriate set of boundaries for when their child's life is his / her own...and not become so enmeshed in their child's lives that they don't know where one stops and the other starts. Parents need to be able to draw the line.
A parent being a best friend to their child (ren) is not a bad thing...it's what you do as an individual that determines if your behavior is appropriate / inappropriate
Just MHO