The breastfeeding neighbour has since taken to social media saying she was trying to do the right thing by the baby.
"I currently have a two-year-old who still breastfeeds and I'm trying to wean her off," the mother wrote.
"I recently just had a guy move in next to me with a five-month-old baby girl.
"When I asked about the baby's mother, I assumed she was out of the picture because he replied with 'she's not gonna be here'.
"I offered to help him with moving in and anything else, as casual neighbourly friendliness, and he took me up on it. I helped babysit and set up furniture.
"He asked me if I could also breastfeed his kid because he was concerned about formula and wanted his kid to be strong.
"I thought it was kinda weird but I agreed, mostly because I took pity on him.
"He really didn't seem to know what he was doing with the baby and I thought that maybe I could help at least somewhat."
When the baby's mother finally showed up, she blew up at the neighbour.
"She said that I was a sick person to 'let some random baby suck up my bodily fluids' and I wanted to destroy her bonding with the baby and alienate her from her baby."
Even after explaining the situation, the mother told the neighbour she was "weird" and not stable in the head because you should never "breastfeed someone else's baby".
"She said that there's formula for a reason and what I did is unacceptable."
The incident has divided many online, with some saying a line has crossed.
"Breastfeeding is something really personal, emotional and intimate and if I was a mother I'd be furious if some stranger had breastfed my child without my permission," one said.
Another added: "In my opinion breastfeeding a stranger's baby without the mother's permission crosses a line."
However, others heaped blame on the father for asking in the first place and praised the neighbour for stepping up for the baby's health despite the situation being strange.
