My DD was in the same situation as suishabelle's dd, Small cul-de-sac but mainly girls, One girl inparticular 5 years older than dd took a dislike to her.
My DD makes friends easily so knew everyone in the area. This older girl who is 11, would do exactly the same invite all dd's friends into garden but not dd so she was often left alone. She would also pull dd's friends away to whisper and name call dd, she got her younger sister to paint my dd's brand new clothes which were ruined and then it turned physical. She once got a group of four to take turns riding over my dd's feet with thier bikes. There was also alot of pushing and hitting. She got very nasty
Now what i think the problem is was this girl and her sister are not allowed out to play unless they are being supervised by by an adult, (dad has to potter about in the garage) even though all the kids can be seen from the window. And would often have to be in bed at 7pm where as the street kids are still out playing in the summer. The girl would be at the window watching them when supposed to be in bed. I kinda felt sorry for her.
So when she was allowed out she was rebelling against parents strictness, and my dd was bearing the brunt of her fruatration. Maybe because she has lots of friends,i dunno
But no matter what this girl did to dd,dd would always still try to be friends with her and has built up a thick skin over the last couple of years.
it came to head a few weeks ago when dd had her pencil box and paper out in the street when the older girl delibrately ran over the box on her bike destroying it, screwed up dd's pictures and put her pencils down the drain. Dh went out and asked the girl why she did that. Her dad realised something was going on and came over Dh explained what had happened and what had been going on. Girl was taken in and grounded then brought out to apologise to dd.
it could be that there is something else going on with this girl and your ds is suffering for it. If you are still on talking terms with the mum then it might be worth explaining to her why your ds is so unhappy. As she may not have clue whats going on.
With regards to ds, does he have one friend you could encourage him to spend time with in your garden/house away from the others.
If she sees he's not bothered then may back off.