Kids - Being mean over message

[Llewey78]

Dd is friends with a little girl same age (9) lets call her TJ, their friendship is temperamental.
TJ has an older brother (11). The girls got into a nasty argument over message, they were equally at fault. However, the brother then got involved and threatened to beat up Dd.
Aibu to think this is a step too far?
The mother doesn't seem to be showing any signs of apologising on his behalf. We agree the girls were equally at fault, but that does not sit right with me.
Why on earth an 11 year old thinks its ok to beat up a girl is beyond me?!
I absolutely will not be allowing this 'friendship' to continue as i was already sceptical due to TJ discussing things with Dd that were imo far beyond their years, perhaps down to having an older brother, but this is the nail in the coffin for me.

My first question is why does a 9 year old have access to either messages/text or phone/computer?
As you've said it's no ok for an 11 yo to be threatening a 9 yo but here you are.
Just tell your DD to stop swing friend but then 9 yo friendships are very mercurial.
Definitely stop her access to messaging on technology she's not old enough!