Amid the sea of Nigella coverage, beneath the giant photos of her walking gracefully into court with her head held high, and the flashback shots of Saatchi with his hand round her throat, there was a small, drop-in picture that made me smile like you wouldn't believe.

Ah, there he is: John Diamond. The invisible third player in the drama. The "tragic first husband".

I'd guess from the polo-neck that this photo was taken after his tracheotomy. I can see from his low body weight that it's after he lost his tongue, after the tube-feeding, after he started communicating with strangers by notebook and pen. (I could still understand his speech pretty well, because we spoke often; Nigella understood him perfectly; and his Mini-Me toddler children, with the fluency of all small people, had no problem at all.)

So: that picture is deep into the throat cancer, after his body has been ruined; after the doctors have bit by bit taken hope away.

And yet, look at him. Shining like his name, twinkling with confidence and mischief, got up in a dandy purple suit to fit his reduced frame. You can't see his feet, but I'll bet you £100 they were shod in something buckled and snaky.

Nigella is next to him. She is the Domestic Goddess; she's the one with film-star looks and growing film-star legend; she is the healthy one who's going to survive. But he looks like the one in control. She is shy and camera-nervous on the arm of her passionate, brilliant, adoring and challenging husband. He didn't always make her life easier, but he was so proud, so protective, and so profoundly in love with her.

Having collaborated with John Diamond and the writer-performer Robert Katz on a play about his life and work, I'm the closest thing John had to a sort of biographer; I wanted to write about him, this week, for those who did not recognise the name "John Diamond" when it came up in the coverage.

When people read, last Thursday, about a terminally ill man acquiring cocaine for "some escape", they might have imagined something rather lame and desperate, weak and hopeful. John mustn't be remembered like that.

He roared in the face of his diagnosis. He got tattoos. He went gambling in the afternoons. He bought a Harley Davidson. He tried various drugs, prescription and otherwise. He decanted glasses of champagne into his feeding tube. He out-risked risk itself. As Marvell wrote: "Thus, though we cannot make our sun stand still, yet we will make him run."

I've never seen Nigella take drugs. In 15 years, I've only once seen her slightly drunk. It's hard to think of anyone I know for whom the expression "off her head" is less appropriate, except, perhaps, my mother-in-law.

But I'm not surprised to hear she tried coke when John was around. Me too. The only time I've ever tried cocaine – which is also the only time I've ever seen cocaine – was under Diamond's terrible, twinkling influence.

John was amused that a purported "media figure" such as myself had never sampled or seen this apparently ubiquitous social wheel-oiler. He was also tickled that an all-night gambler like me was so square and nervous about drugs.

One day – and it really was during the day, at some lunchtime party – he insisted I try it. He was in a place beyond rules, the Twelfth Night that can follow a death sentence, and he enjoyed being the emperor of an upside-down world.

Nigella wasn't there. She came to the casino with us once or twice; I think her approach was to join in just enough to experience that side of John's terminal life, while keeping the home fire burning and maintaining structure for their adored children – who have, by the way, grown up beautiful, sweet, clever and capable. Don't worry about any emails you might have read; we should all raise such terrific kids.

"I'm not snorting it," I said. (Like most geeks, I suffer a fabulous range of breathing disorders.)

"Rub it on your gums!" shrieked John, gleefully.

What are you going to do? This was a beloved, dying friend. Decadence was his cushion. He was facing something enormous and terrifying, that he could only face alone. But if he invited you to join him in the decadence, how could you say no? Maybe he'd feel less lonely for a moment. If I felt like that – and most of his friends felt like that – how on earth must his wife have felt?

He knew it too, the cheeky sod. When I'd done too much money in the casino and decided to stop, he'd twinkle: "Put it all on number 26… it's my last wish…"

When I told him I would never ride a motorbike, he insisted immediately that I hop on the back of his. I am not a weak-willed person. But somehow, there I ended up, on the back, clinging to him for dear life, yelling: "Go slowly! Go slowly!"

So, I rubbed some of this powder on my gums. After a bit, my mouth went numb. There was nothing exciting about it. It was just like going to the dentist. John was disappointed, I was relieved, and that was the end of that.

When you try to picture the "tragic first husband", do not picture a dying man. Picture the most living man there ever was. Read his book, if you can. He was funny, angry, rebellious, ingenious, ambitious and unforgettable. Twelve years after his death, he's still more alive than half the people I know.

Throughout all this nonsense, I have felt him kicking furiously – and I hope his family has too. I hope his children know him; he could not have loved them more. He was always, and always will be, #TeamNigella.

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