Actually - I went through something quite similar. But the little boy was always targeting my DS, he was so agro and really quite violent towards him. To the point of scratching him, trying to scratch his eyes and pulling his hair. The other mother just would never do anything. And if she did it would be a "don't do that sweet heart" give him a cuddle type thing. It got to the point where my DS was literally terrified of this kid, he would start to shake whenever we would see him and he would hide. The mother thought her child was perfect, amazing and overly intelligent. It drove me crazy. Anyhow - one day I caught this little boy alone in my 4 week old twins room trying to smother one of them. I told the mother and she did the whole "sweetie, we don't do that".
I basically had to cut myself off from them. It sounds harsh - but I didn't trust her child and I didn't trust her to do the right thing either.