She said her husband described the man's reaction as "completely disgusted" with his lips curling in disapproval.
The man then said: "Not really the right place for your daughter is it?
"This is the male change room, not the female. She shouldn't be showering like that in here."
The mum wrote the man's tone was critical and the exchange left her partner shocked.
As the man went into a toilet cubicle, the father and daughter hurriedly dried off and left the pools.
Discussing the incident later that night, the dad asked his wife if what he had done was inappropriate and said he was second-guessing himself.
"I was completely confident that what he did was fine, normal and practical," wrote the mum.
"Instead, I became absolutely infuriated at this man. Not only was I infuriated that he had made this remark to a father caring for his daughter but infuriated that now my husband was insecure in his own parenting abilities and the decision he made.
"'How is a father helping his six-year-old daughter shower in a changeroom after a swim wrong?' I replied to him.
"There is nothing inappropriate about it, especially in this particular context."
She wrote that her partner had taken the most responsible option and she was "hurt" for how he been made to feel by the other man.
"I understand that everyone comes with their own views and beliefs and perhaps if Isla was older and could have gone into the female change rooms unsupervised I would have understood where this man was coming from.
"But she is a six-year-old girl and nothing about her using a public changeroom, even if it is the men's, with her father, is wrong."
