To speak to BFF-turned-bully's mother?

[herbaceous]

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.

An update. Speaking to the teacher actually helped, they had circle time about 'respecting each other', and an eye was kept. But then the teacher left. To ensure continuity I met the new teacher, who is new and rather wet and ineffectual, and I fear XBFF mum got wind, as she started being very frosty.

I was tempted to have many words with her, but previous forays down that road had not ended well. When XBFF hacked into DS's roblox and sent rude messages, that was apparently DS's fault for giving him his password. When XBFF hit him in the goolies in ukulele club, that was 'just high spirits'. This from a woman who proclaims parents should always take responsibility for their children's misbehaviour.

DS has made a new, rather unsuitable, friend, that XBFF doesn't like so won't steal. (Scary mum has annexed other potential friend). XBFF has dialled down the nastiness, but was doing that maddening mimicking of everything DS said, so he unwisely said to his new friend that he 'hated' XBFF. Who then went crying to his mum after school.

Ever since then she has not been talking to me, and many mums of their friendship group are also being extremely frosty. GAH. So annoying. I am dealing by smiling sweetly and pretending I haven't noticed. But so tempted to give her both barrels.

I have NEVER fallen out with anyone in my life, whereas she has a list as long as her arm. Yet I'm the bad guy. RANT RANT RANT.