I have a very hard time believing that this mum actually does nothing/thinks nothing of the bad behaviour
Like "there is no such thing as a two year who can do an evil stare and show every sign of hurting, becuase the hurting is the intent" the fact that something is outside of your own experience does not make it non-existable.
google Taking Children Seriously, see also Radical Unschooling, those are just two of very many theories where some (not all, some parents and kids do very well and turn out lovely on this sort of parenting stratagy cos it suits their temperaments and the parents do it well) practitioners take avoidance of "coercing" their child to the point of no action and reaction regardless of what they are doing. I've even had a real live actually in my living room person tell me that it is less harmful to let children have their teeth rot (natural consequence that child can take on board, or not, if desired) than it is to insist on teeth brushing. If somebody is prepared to allow their own child to rot their teeth or refuse life saving medicine, then is does sort of explain why another kid getting clumped doesn't really register as a big deal.
People who believe they will damage or limit (in a bad way) their child if they so much as have an expectation (as in have a thought in their head, not actually verbalise it) in terms of behaviour, exist.
For them the idea of twitching when their kid does something unsociable, let alone intervening, just wouldn't be considered suitable, desirable or helpful.
I don't think fuck her to the mum. I'd rather hope she isn't the type to rationalise and reframe the cat's bums mouths and rejection they are going to keep coming up against as "persecution of the free thinkers by the sheeple who sausage factory their children into mindless automatons, they are just JEALOUS! and I make them feel inadequate in comparison, so they are nasty to me cos they have ishooos" cos then there is the possibility that she'll get less sure of the believed perfection of the style and start to use parenting theories as tools instead of chains. In which case she and the kid might have a better time of it in the longer term. As will the people around them.
With any luck she might not be in that deep and it won't be so much an adopted philosophy, and more of a sentiment or sensation where she can't bear to upset him by telling him off, so has rejected reacting to behaviour as a strategy to avoid having to be "unkind". I think that might be easier to step back from than a sense of identity that has become entwined with a parenting theory. Especially if it's one that came with promises of fabby outcomes "if you just trust your kid" and praise for being clever and special enough to pick it.
I give the OP credit for hanging on in there a year without exploding with repressed frustration, pissed offness and end-of-tetheritus. If my son (thankfully much older than pre school age) hadn't turned around as we left our "special interest" group muttering "I don't want to come back, please don't make me" I reckon I would have got to the six week mark max before coming here and spluttering wildly in enraged, expletive and hyperbole infested incoherence. And then I would have descended into utter atomised rage further down the thread as you all accused me of being horrible and utterly unreasonable. 