Growing up, your parents’ friends can feel like extras in the drama of your life: there, but peripheral to the action. Les was that for me. She was my mum’s best friend: the giver of the ceremonial birthday £5 note and bestower of strange charity-shop finds. We rarely lived in the same place. While I was in Yorkshire, she was in Bristol, and she moved north only when I went south to university. We would share a hug and a chat a few times a year. I liked her, but our paths rarely crossed.

Then my lovely, huge-hearted mum died. It was a violent freak accident in a foreign country, and her death was a catastrophic rip in the fabric of our family. I was 29 and pregnant with a confused and understandably demanding one-year-old son. My sister was barely 18, my stepfather quite unwell and spent his days making and receiving endless unbearable phone calls as the news reverberated around my mum’s sprawling network of friends, family and colleagues. Her many siblings flocked round, filling the house with comforting but stressful activity – small dark women I would cross on the stairs who looked so like her, but weren’t. We were lost in the twilight-zone bureaucracy of sudden death; a time when there was nothing and everything to do. It didn’t even feel like grief (or how I had imagined grief before experiencing it). We were just stuck in a horrible, frightening place I hadn’t known existed.

Into that stultifying fog came Les. I remember the physical sensation of relief I felt at seeing her; her hug felt like oxygen. “Shall we go for a walk, Em, just us?” she said, and we did, walking slowly, arm in arm, around wintry York. It wasn’t at all that Les stepped in and made things better. She had just had a bad car accident and was in great pain; she was in shock as much as we were. Rather, she knew how to sit with the awfulness. Like many inveterate worriers, Les is great in a crisis: when the worst happens, she was sort of expecting it anyway.

We both missed Mum terribly, but our bond seemed to soften the sharp edges

I can’t remember exactly what we said to each other that first day (I think we talked about what to call the new baby when it arrived, as well as the bare, grim facts of the moment), but I knew I could say absolutely anything without shocking or upsetting her. That created a sort of breathing space that allowed me to move on, just fractionally, to whatever was next.

For the next year or so, Les helped more than almost anything else in small, but utterly reliable, ways: a sneaked cup of tea, or a frank, rambling phone call that got straight to the bare bones of things. Pain. Anger. Dread. Then round again, without boring or depressing each other, somehow. Neither of us slept much, so we would text in the grey early hours, a tiny flash of connection in every blunt message. We both missed Mum terribly, but our bond seemed to soften the sharp edges of it. She was somewhere in the ether between us.

When my younger son was born, four months after Mum died, Les came almost immediately, arriving in the hospital the next morning. It’s an almost painfully pleasurable memory: she and her wonderful husband, Rob, holding my tiny son, and beaming with almost grandparental pride. If anyone could understand how joy and grief can coexist, can heighten each other, it is Les. Lots of rotten things have happened in her life, but she has a capacity for pleasure – dancing at a festival, a good bun or a trip to Whitby for fish and chips – that delights and inspires me.

‘Somehow, this friendship is effortless. We are both gloomy and anxious, but make each other laugh.’ Photograph: Gary Calton/The Observer

I moved to Paris soon after that, and Brussels a few years later. Sometimes, we would be out of touch for months or even longer, but by then we were friends and it was quickly clear it was not a high-maintenance friendship.

I can’t see how we would have been friends if it weren’t for Mum: Les is 72, and survived a tough upbringing in Wales to become an academic, working in medical education and research. She is deeply connected, with a vast network of friends, family, acquaintances and Rob’s music-playing cronies who drift through her home and life. I’m 44, a cosseted middle-class introvert who went into corporate law before escaping into writing, with a tiny group of friends and a prickliness that means I struggle to make more. Somehow, though, this friendship is effortless. We’re very alike in other, bigger ways: we’re both gloomy and anxious, especially about our kids (two apiece), and we make each other laugh with our wild catastrophising – every conversation with Les includes at least one lengthy anecdote about someone I don’t know struck down by terrible illness or tragedy. I never feel I’m being too negative with Les, even though I usually am, and nothing creates space in my head like an hour with her.

We share lots of pleasures, too: books, consumed voraciously and exchanged; a lazy afternoon at the Turkish baths; people-watching. “We both find people enduringly fascinating,” says Les, which we do: we once spent a full, deeply satisfying hour speculating on a wedding party seated in a cafe behind us.

Our friendship survived for years nourished by emails, snatched hugs in station cafes and the occasional proper visit. She and Rob parked their camper van on our Brussels street and did tai chi in our back yard one year; another time we accidentally coincided in New York and went for celebratory martinis at the Algonquin. She sent my sons birthday cards (plus €5) in schoolgirl French and brought me one of her grandmother’s silver spoons when I got married; I sent her books, especially when her health was bad.

I can’t give Les what my mum gave her. Theirs was a once-in-a-lifetime friendship but what we have is an unexpected gift

Then, in a lovely moment of serendipity, we moved back to York last year, within weeks of each other. I still can’t quite believe my luck at the astonishing, simple joy of just dropping in on each other, at last. Les and I go to the cinema and raid local garden centres; she and Rob bring their two delightful grandsons round to see our tortoises or eat cake. Best of all, we can just meet up on a street corner to mutter darkly at each other for five restorative minutes when the need arises.

Les is family in the most essential way, but our relationship is not a mother-daughter one at all. Part of its strength and joy comes from not being freighted with all the intensity and ambivalence those relationships have, the expectations, the hurt and the things unspoken. “Not being related makes it easier,” Les says. “You can have a good moan without rocking the boat.” She is, though, a sort of grandparent to my children. Her love of, and pride in, them fills an empty space for me: there’s something oddly grounding about the continuum of birthday tenners (inflation!), Advent calendars, Easter eggs and terrible charity shop finds. She’s never shaken that habit, but she knows her own weakness: “Straight in the bin, Em!” she exhorts, handing over a yo-yo, a salt cellar shaped like a camper van or a crocheted llama.

I can’t possibly give Les what my mum gave her: theirs was a once-in-a-lifetime friendship. I wish she still had that, just as I wish I still had my mum. But what we have is an unexpected gift. It doesn’t fill the hole, but we have bridged it, spinning skeins of affinity, memory and love between us. “We need that little silk thread of connection,” she said, when I called her to talk about this article (we swiftly moved on to exchanging tales of disaster and plotting our next garden centre trip). We lost something irreplaceable, but we gained each other. Inherited each other, even.