Debbie Hayes was found dead on the bedroom floor of Jason Elliott's home in Hassett Close, Preston after a night of drug taking involving the couple and another man.
An inquest heard that it was a mystery how Debbie came to have fatal levels of heroin substitute methadone in her system.
Her family told Area Coroner Chris Long that although she had been a regular heroin user for long periods they thought she had been "clean" for the last four years.
And Mr Elliott said Debbie must have taken the methadone after he had gone to bed leaving her in the lounge with his heroin addict friend called Mark. A bottle, for his own personal use and normally kept in a locked box, was later found empty.
"I think he must have said to Debbie 'come on let's have some methadone,'" said Mr Elliott. "I wish I'd never invited that lad into my house. I'm sorry for what happened.
"I’d just like to apologise to the family. This is a tragedy that didn't need to happen. I wasn't aware she had taken methadone. I have to live with this for the rest of my life."
Mr Elliott, unemployed, described Debbie as "loving and a gorgeous person." He told the inquest he had taken her in two years earlier at the start of lockdown because she was living on the streets.
"We grew pretty close. In fact, the week before she died she asked me to marry her."
The day before her death Mr Elliott and his friend travelled by train to Manchester to buy drugs. They bought quantities of valium and pregabalin tablets.
Back in Preston he said he knew Debbie had “a couple of valiums,” but didn’t know about the others. "It is quite possible that Mark was giving her some because he used to take them on the hour, like Smarties.
"I didn't see Debbie taking any drugs that day and I certainly wouldn't have given her any methadone if she'd been on valium."
He said he went to bed before midnight, leaving Debbie in the front room. He woke at 6am and she was on the bedroom floor fast asleep. He checked her a couple more times in the morning and she woke both times. But she was found dead when her daughter called at around 11am.
Post mortem toxicology tests found methadone in her system at potentially fatal levels. She also had other drugs, but not in excessive quantities.
DS John Whittle said there was no evidence to suggest the death was suspicious or that she had been forced to take anything against her will.
The police found drug paraphenalia in the house, but no needles. The friend Mark refused to give a statement to officers.
Reaching a conclusion of "drug-related death" Coroner Long said: "I have no evidence that anyone directly saw her take drugs. There was a history of Debbie taking drugs in the past."
"There is no evidence to suggest she didn't take those drugs willingly or there was any third party involvement."