My DS, I am sure, has some form of autism. After seeing him for five minutes, the professionals decided not. But when I live with him, I do think so. He wasn't invited to parties, he didn't have parties. The only one he had was when he was about 6 or 7 and he ended up upset upstairs because he found it difficult to mix with people. He has only just found mates, he says, that he feels "at home" with and now is worried that he won't find the same at university. I do try to reassure him but he will still worry. It is awful when you feel like that, I did when I was a child somewhat. He has to work very hard at making friends although he is very loyal and trustworthy when he is a friend.
My ED, on the other hand, had the whole class to parties and was invited all the time to parties. It's just different personalities and sometimes, being easy to make friends can be as bad. It can be a shallow friendship, because they are easy come, easy go. But my DS cherishes his friends now, and works hard to keep them.
red Maybe if this is what the mother is teaching her son, then your DGS would be advised to find others, as well as this boy, to play with at school. Maybe not put all his eggs in one basket and widen his net a bit to others who would appreciate and care for him for who he is. Maybe his mum can have a word with his teacher and help him to make other friends, sit next to new people, broaden himself a bit. He will need to learn how to deal with this as he gets older, so this is a life lesson, unfortunately, very sad. If someone was being nasty to us, as adults, then we would move on, cut our loses. I'm not saying not to play with him, just have him play with more people too so that he has more choice and therefore less hurt in the future. Just a thought. Keep your chin up. 