I'm probably going to get flamed for this, but I deliberately cut contact with a very good friend, purely on the basis of her DC's behaviour.
I met her at playgroup when my DD and her DD were about a year old. She had a DS of 10 too. For about 3-4 years we did loads together.
She didn't have the same approach to parenting as me- rather heavy on the junk food and TV whereas I am not- but frankly I couldn't give a shit, we went to soft play, petting farms etc, lots of lovely days out.
Long story short, the older the son got, the stranger his behavior was. He did have some mild SN, but was in mainstream school, and it went beyond SN. He watched a lot of late night TV and played grand theft auto etc, and began to develop a fixation with violence, to the point where he deliberately crushed a pet guinea pug with a breezeblock to 'put it out of its misery' after a fox got it. 'It made him feel good' apparently. He also became increasingly violent and aggressive towards his mother and sister, and other, smaller DC
I feel horrible saying it about a kid, but he made me feel very, very uncomfortable. Unfortunately his behaviour filtered down to his little sister, whom he bullied terribly- and this of course, had an effect on my DD.
It came to a head when the little girl, on a visit to my house, hurt my DD quite badly, destroyed some of her toys, and threw things at me when I remonstrated with her. The whole time, my friend just sat there, saying nothing. I had to ask them to leave, as my DD was distraught.
Friend tried to get in touch/meet up after that, but I just found excuses or ignored her. I couldn't have my DD subjected to that in her own home, and it was becoming increasingly apparent that there was something not right happening in that family.
Sure enough, a year ago, I found out something which confirmed my suspicions.
I still miss my friend, and I miss the lovely little girl her DD used to be, and the times we had, but honestly, I was out of my depth with that situation- it got to the point where before every meet up, I was apprehensive and anxious, and I was not happy with my DD being in the company of her children, because frankly, I didn't know what they would do next.