Actually, this is a complete non issue - OP has a kid of an age that he's allowed to be in the women's changing room, according to the pool policy.
So at this point - yep, I'm with her as far as if I swam there I'd be aware they allow 8yo boys into the female change. If I didn't want to change with 8yo boys I'd go elsewhere or use a cubicle (whatever my private thoughts on whether this is a reasonable policy).
She's also said she gets that she'll have to make alternative arrangements once he turns 9.
So all good, albeit OP's tone may have been somewhat abrasive.
For me, the more interesting question is why she can't just go into the men's change to help him. He's a male. He needs a carer. She's that carer.
Because she doesn't want to look at random penises? Fair enough. Wouldn't bother me.
Or because the chaps could reasonably object to a woman in their male space? Also fair enough.
So if we are saying that a) women & girls shouldn't be obliged to look at naked males if they'd prefer not to, & b) men & boys have a right not to be discomfited by females when they're naked...then OP's ds once he reaches the cut off age, & all other boys once they do, do not belong in female changing a) because w&g don't wish to see him nude & b) he has a right to privacy.
So however you slice it, the OP's son shouldn't be in female changing after he turns 9.
Which - & again this is where we came in! - she seems not to be arguing against. ..