Bit at a loss.
Our DS has had the same school BFF since year one - they're now year four. They used to be like peas in a pod, in a larger group of friends, and all was lovely.
However, a few months ago BFF started getting very bossy with DS, dictating rules in games, telling DS which bin to use, how long to spend on his school work, stuff like that. DS is very amiable, so kind of went along with it.
Inevitably he eventually got fed up with it, and started saying 'no'. At this stage BFF started to hit him, shout 'don't be so rude', or threaten to 'get him blocked' off their online games. He also hacked into DS's roblox game and sent rude messages to his friends, stuff like that.
It has got gradually worse. BFF has 'accidentally' kicked DS in the back, tripped him on the stairs, etc, and now if DS gets angry whispers to their other friends about him, and takes the mickey. He tells lies about what DS has said.
DS now tries to ignore him, but BFF has started to annexe their other friends, and say horrible things where DS can hear but not see. When DS turns round, both deny it. 
I have had a word with their teacher, who says I'm not the first to mention this and that he's keeping an eye and that DS can always talk to him. However, the BFF is cunning and most incidents happen away from the teacher.
Should I speak to the mum? We had been good friends, though have cooled recently after she lost her shit about me picking DS up from a sleepover too early. She is pretty scary.