That's it really. DS has a friend, who is bullying DS best friend, and DS is neither sticking up for him or reporting it to the teacher. He is just turning a blind eye to it. This is very nasty bullying, both physical, hitting repeatedly, kicking and psychological, nasty comments, humiliation and embarrassing the other child. His Mother has spoken to the school and school are aware and have said that they will take action, but I don’t know what form this has taken, as the bullying behaviour is still happening.
I have challenged DS about this and what is the right thing to do and he told me that he is frightened that bully won't be his friend anymore if he sticks up for his other friend or tells the teacher.
The child who is being bullied is lovely, and not the sort to retaliate or defend herself. My DS is his only real friend. Outside of school they are inseparable and very close friends, they never have a cross word with each other.
I do wonder if the friend is jealous of DS and his best friends strong friendship and that this may be behind the bullying behaviour.
I was bullied at school, so this really strikes a chord with me and I totally feel for DS best friend being on the receiving end of this and how wretched he must feel that none of the other children are sticking up for him, including his best friend. It makes me feel so sad and so disappointed in the behaviour of my son.
By witnessing this behaviour and doing nothing to stop it, I feel that my DS is at the very least condoning the behaviour. The bully is always encouraging my DS to join in with the bullying of his best friend and I know that on one occasion, he has actively joined in with it, which disgusts me.
My DS is lovely, he is normally very kind and caring but also very easily led and influenced by other boys with stronger personalities, like the bully. DS wants to be popular and "cool" at school.
How do I get DS to see that this isn't the right attitude???!!!
Please recommend any helpful books or experience on resisting peer pressure.
I have told him that "strong people stand up for themselves, stronger people stand up for others" but he says that's OK you, I'm not that strong! 
and that he is scared to take a stand against bullying in case he loses the bully's friendship (which I would be highly delighted about, as he gets DS into trouble at school)