This is not completely off base. Prior to getting together with Yoko, John was addicted to LSD, not heroin (that fact that he was already a drug addict is one that bashers prefer not to acknowledge). If my recollection is correct, Yoko had tried heroin before while John hadn't. Shortly after they became inseparable, they both became addicted.
I believe Yoko's story is that John asked her if she'd had heroin before, and she said yes, and he suggested they do it together. I take this version with a grain of salt - Yoko is not always honest, and in this case I can't even blame her.
But I find it unlikely that Yoko would have needed to do much more than make the suggestion herself. I doubt she would have needed to do any convincing or put much pressure on John. John Lennon was quite down with dangerous and illegal drug use.
Although, perhaps that doesn't matter when judging Yoko. After all, Eve doesn't seem have had to do much convincing to get Adam to eat that apple, and she still needed to suffer in childbirth and have Adam rule over her as punishment.
But regardless of the circumstances of the start of John and Yoko's heroin addiction, they started trying to get off it pretty quick. John and Yoko get together in spring of 1968. Cold Turkey was released in fall of 1969. It took a year and half at most for the two of them to decide they didn't want to do this anymore. A highly stressful year and a half, at that.
In contrast to all the speculation and blame of whose idea it was to get on heroin in the first place, I really can't remember reading or anything anything about which one of them first thought to stop. But based on what I know about their personalities and their relationship? I'd put my money on Yoko. John was not the type of person to decide a drug was ruining his life and just stop using it. And Yoko could be controlling of John in some ways, the bashers are right there. Of his diet, for example. I've also heard that Yoko was the one to suggest primal scream therapy. Yoko's ideas were frequently nuts (macrobiotic diets? primal scream therapy?), but she was trying to keep herself and John physically and emotionally healthy. She was undoubtedly aware that John was self-destructive, and she was trying to keep him alive.
Barring any contradicting evidence, I'm going to assume the going cold turkey on heroin - both the original idea and the implementation - was on her initiative.
I read an except from a book called Cornflakes With John Lennon, written by a reporter named Robert Hilburn. It's just some anecdotes about his experiences meeting celebrities. I'm having trouble finding a readable version of the excerpt online now, so this will have to be from memory. Hilburn met John Lennon in 1980, around the time Double Fantasy was released. One of the things he had wondered about, was whether John was still abusing drugs. John assured Hilburn that he wasn't. The reporter described a point when he saw John sneaking something from a cabinet to eat. Hilburn thought it might be drugs. It turned out to be chocolate. "Mother", John explained, didn't want him eating chocolate.
When he heard that John had been killed, Hilburn thought about how John's death undermined one of the very things John had worked hard to achieve: not being a rock and roll casualty. Hilburn is the only commentator on John Lennon who I have seen make that point. John's post-Beatles writing frequently alludes to a determination to survive, to stay alive, from Lennon Remembers, to Scared on Walls and Bridges, to a song-in-progress called Help Me To Help Myself that I first heard as a bonus track on Double Fantasy. But this is not what John Lennon is remembered for. Instead, he's the troubled genius and martyr - the very martyr status that he insisted he didn't want.
Yoko Ono is a troubled and often mean-spirited person, not really more emotionally mature than John Lennon. She was intensely insecure about her relationship with John, and felt deeply threatened by most of the people who loved her third husband before she knew him. But she also worked hard to keep him alive, to keep his self-destructive tendencies in check. And I think that is part of why John kept on coming back to her. John Lennon was many things: An abusive asshole, a cruel and neglectful father, a brilliant musician and songwriter, a selfish but frequently warm person. And he was also a survivor.