A fatal accident inquiry heard Kimberley Hainey was "always" drinking whenever she spent time with neighbour Catherine Rowand, 37.
Ms Rowand said she and Kimberley would hang out at Rowand's home in Paisley around two or three times a week in early 2009.
And the mum said that Kimberley would never be without alcohol.
Kimberley's toddler son Declan was found dead in his rubbish-strewn cot at the flat in Bruce Road, which was above Rowand's home, in March 2010.
His body was so decomposed doctors could not say for sure how he died and a recent post mortem examination states the cause of death is "unascertained".
At the inquiry into Declan's death at Paisley Sheriff Court, Rowand said: "Kim would always have alcohol - her wine.
"It was just one or two, social."
When procurator fiscal Stuart Cassidy asked Rowand why Hainey spent so much with her, she said: "I just put it down to her being lonely as she didn't have anyone else round about.
"She wasn't always invited down, sometimes she would just appear when she knew I was coming in from my work."
Rowand, who was working in Pizza Express at Braehead at the time, described Hainey as a doting mother when she moved in above her.
She said: "When she first moved in she changed Declan four or five times a day.
"He was always clean and tidy and well presented.
"The flat was always really tidy. Any time I was there she would be tidying round about.
"She made sure everything was put away nice and neatly."
The witness said that her friendship with Hainey changed in May 2009, a month after Declan's first birthday.
She said she fell out with Hainey after the 38-year-old said Rowand's husband had hit her.
Rowand said Hainey had gone to the house when she was out at work and had a few drinks with her husband.
And the mum said her husband phoned her at work asking her to come home as Kimberley wouldn't leave.
She said: "Graham wanted to go to bed. Kim had had a few and she refused to leave.
"She had a cut on her head, Graham said she'd slipped and hit her head off the door frame.
"Kim told me that Graham had hit her.
"We had been married for four years and Graham had never put his hands on me.
"I believed my husband more than Kim.
"From that day we stopped going to each other's houses."
The inquiry, before Sheriff Ruth Anderson, continues.