At university in New York, Sarah and Zoe were two of my closest friends. I glimpsed them in the first, horrible days: they were a glamorous, laughing pair who crashed every party, sang along loudly with the songs and had no qualms about dancing with each other. Inseparable since childhood, they shared mannerisms and the same way of talking: deep voices with a slight Valley Girl lilt. Both were intelligent, articulate and, above all, hilarious.

Zoe was unsure of her ambition, but Sarah knew exactly what she wanted to do with her life: she was a painter. She studied English literature to please her parents, but painting was her vocation. She turned up late for everything, her hair and clothes spattered with paint, her fingers stained into an artwork of mauve and ochre and vermillion, a lazy smile on her face and the lame excuse that she had lost track of time in her studio.

Zoe was fastidious, but endlessly indulgent of her oldest friend. Theirs was a symbiotic relationship: it was two years before I spent time with either of them on her own.

In retrospect, the paintings Sarah produced in her final year should have alerted us. I sat for a portrait and was alarmed by the dark and misshapen figure I appeared on her canvas, huddled against a bile-coloured background that bore no resemblance to the white light of her studio. We all noticed that she seemed wilder than usual, but we were all a little wild: it was a frightening time, our encounter with adulthood imminent, and we reacted with childish irresponsibility.

After college, I came to England to study some more. As it happened, so, for a few months, did Zoe. And Sarah, who was backpacking around Europe, stopped by. On my 21st birthday, just after I had arrived, alone, in a strange place, Sarah and Zoe swept into town as a surprise. For the next three days we whirled about in an exuberant frenzy, periodically paralysed by hilarity and fortified by cheap champagne. I picture Zoe and Sarah, wearing the silly hats they picked up in the market, arm in arm, howling with laughter. That weekend is one of my favourite memories: it is also the last time I really saw Sarah.

I have seen her since, of course, in the flesh. And in fact, by that time in England she had already slipped along the spectrum away from 'sanity'. But we didn't know it, and we thought of her as herself. Certainly she was still more herself than she has ever been since.

It is impossible to tell whether an undiscussed abortion and the months she spent travelling alone precipitated her illness, or whether it was inevitable: the history of mental disturbance in her family was overwhelming: schizophrenia had already struck her mother's mother, her father, her sister . . . .

Within six months of our fiesta in Oxford, Sarah was committed by her parents to an institution in her native California. There she was prescribed medication which she (rightly) felt altered her personality. It was a struggle to regain her freedom, and then her autonomy: once released, she lived with her parents. Unable to work, she was penniless and felt trapped. She hated the drugs, but initially she took them. When she had proved to her family that she was functional, and when they could no longer bear her profound misery, they did as she asked and sent her back to her friends in New York. She rented a flat and, for a time, held down a job.

Relieved to have retrieved control of her life and eager for that control to be absolute, Sarah eventually stopped taking her medication. Six weeks later, frightened by shadowy things she now found lurking in her apartment, she moved out of it and into the living room of a friend's place. He could stand it for only 72 hours: during that time, Sarah didn't sleep. She didn't even lie down. She ranted, and chain-smoked, and ranted some more. She paused to eat everything in the fridge, and then took up ranting again. She had phenomenal energy. She had endless grievances. She had a paranoid plan of the cosmic conspiracy against her: all lawyers were guilty, and all psychiatrists. More than that, her parents were guilty, her friends were guilty. Zoe was guiltiest of all. Zoe, according to Sarah, was responsible for this whole mess.

Her host called Sarah's parents. Her mother flew to New York, and Sarah was again committed. To her, this was simply further proof of the conspiracy. (To this day she maintains that her friend had her locked up for eating his strawberries.) Since that incident, Sarah has resisted medication and institutionalisation - indeed, any psychiatric help - with superhuman fury. Trapped in the bleak, incomprehensible world she inhabits, she has trailed, filthy, through the gutter; turned tricks to keep afloat; married a foreigner for money; and given birth to a child whose father she could not identify. She is fighting to be herself.

But that self is so distorted that none of us can recognise her. Worse, we can do nothing to help. Following threatening phone calls and a barrage of insults, Zoe has cut herself off entirely from her childhood friend, fearing for her safety. We are all sworn to secrecy about Zoe's whereabouts, and have promised never to mention Sarah in Zoe's presence. Sarah has never forgiven the New York friend who called her mother.

For a time, she tried to reach out to her old friends, ringing us from California every other week, wherever we were in the world (I was here). At the beginning of the conversation, she always sounded OK: but it was like an avalanche, violent, inexorable, until you finally escaped, an hour and a half later, drained and angry. In time, we all took to screening our calls, closing our ears to Sarah's jaunty, then bitter, pleas on the answering machine. When she found out that I had married without inviting her to the wedding (how could I? Zoe took part in the service), she stopped calling altogether: I still feel my failure acutely, and her silence is more damning even than her messages were.

Not one of us, her friends from university, has managed to stay in touch. Her struggle not to be institutionalised has left her isolated and enraged, and, tragically, at every turn her paranoia is only reinforced. But we were condemned to fail: it was those closest to her who were implicated most quickly in the plot she perceived around her - her parents, her oldest friend. The general condemnation radiated outwards from there.

Everyone who knew and loved Sarah now carries the burden of our inability to do anything for her. Wounded by our inadequacy - which she can only see as betrayal - she has dismissed us all. Whatever happens henceforth, Sarah goes forward alone and angry, deprived, by her illness, of the people who loved her most.

The promotion of 'care in the community' (which Sarah, in her search for independence, would most avidly support) raises the question of what community, exactly, can care for the mentally ill. If their families and those who love them - and this must be any individual's 'community' - cannot, who will? Are people like Sarah doomed forever to misery and isolation? One cannot but wonder whether more sustained, properly supervised medical care, if provided early and over a longer period - even in contradiction of Sarah's wishes - might have helped.

Mourning the loss of her best friend, Zoe, at least, believes it might have. And now knows what she wants to do: she is training to be a psychotherapist.