@verysmellyjelly - I actually thought you were agreeing with me with your last line - "There is a related point, which is that linking conditions to MbP accusations by implication and in a generalised and clumsy way is actually harmful to real sufferers of those conditions".
This could equally be talking about those who are dealing with conditions, and being accused of making it up.
If you go back, I agreed that there may well be something wrong, but the insistence (by a lay person) that all those things add up to Mbps is a bit dramatic. I think asking someone to take a look is one thing. The point of this condition, surely, is that the parent is making the child ill, to garner sympathy, money, support etc.
This mother may be behaving bizarrely, but nothing she says implies that the child is being poisoned.
I think a lot of it comes down to the carer's personality. If she was a rich hippie saying her child was banned from eating sugar, could only do yoga, not PE, and how she has this wonderful naturopath, no one would be throwing around this life ruining accusation. She would be seen as having more money than sense, and urge worst that could be said is how sad for the child that they're not allowed birthday cake.
If this mum we're talking about is struggling, agitated, has no one to support her, or make her feel heard for whatever reason, she is supposed to be stoic, demure, a martyr.
She is obviously loud, shares too much, and it's clear that, Mbps notwithstanding, OP doesn't like her and finds her annoying.
God forbid it turns out she and the son are both too some degree ND. Issues with impulse control, not reading the room, not understanding that no one else is interested... ?
Post for advice, say you're worried, but don't start assuming that you know the underlying issues, and at least irl, don't start using labels that can be hard to come back from.