Still a lot of assumptions going on here about what DH said. We don't know as OP doesn't know.
He could have had a perfectly reasonable chat with Friend(mother). (I've had occasional parent talk to me at school gate with/without my DC and on occasion I've chatted to another parent - and it is how something is said. It's not inherently wrong at all, if essential, sometimes face to face can nip in bud).
Friend(mother) doesn't like DH. She knows & likes OP.
OP YANBU to be pissed off with her behaviour. She's escalated without any thought of how to deal direct with it with a family and mum she's known for 10 years!!
We know school contacted OP to say friend made a report but school haven't yet spoken to DH for opportunity to say his side.
We don't know content of what Friend alleged to school as OP hasn't shared it. We don't know if OP believes it, or it's true or exagorared. We know FriendDD allegedly cried but not reason for that. We suspect DH can be abrasive as OP indicates such but not in what context and how he shows it.
We do know FriendsDD lied
And that FriendsDD behaved like a bully/ taunting DS earlier previous day
We do know DS reacted badly, got cross and told FriendsDD that his Mother would "sort her out" (whatever that meant/ not clear how threatening that sounded to FriendsDD)
(We suspect that DC "spat" didn't need much intervention - that OP and family wanted to chat briefly direct to Friendfamily rather than school but know that FriendDD had already involved OP with a lie).
Those are the only facts we know. Anything else... PPs telling OP that her DH is awful and how the Friend was right.... is mere conjecture. It may turn out to be true, or completely false and unfair.
(We have at least two instances of FriendsDD behaving manipulatively).
But it isn't known no matter how many PPs pretend they know for sure!! 🙄