I can't play unhappy families: Gillian Martin finds there are no rules of etiquette to guide a parent at the end of her daughter's marriage

A hotel dining room, and my young assistant and I are having breakfast. She lays a slice of toast across her left palm and butters it with a razor-stropping motion of her right hand. I drink my coffee in order not to say: 'That isn't done, you know.'

At dinner, as she slices her bread roll and goes through the razor-stropping again, she suddenly points at my plate with her knife. 'Why do you do that?' she demands. 'The salt in a little pile and the pepper sprinkled all over? Eh?'

Tight smile, raised eyebrow, keep my cool. Not done to lose one's temper.

'Because that's how it's done. Or so my mother taught me.'

'That's daft.'

Maybe it is; in fact, definitely it is. And it really does not matter if you hold your knife like a pen or refold your napkin or eat your chop with your fingers.

The point is that there are old rules, whether or not you choose to observe them. There are Right Ways and Wrong Ways, and even if the Done Thing was daft, you knew that if you did it there would be no hurt or embarrassment to you or anyone else.

Now I find myself without a lifeboat of established good manners, and all I can do is muddle through, hoping to drop no clangers.

More than 10 years ago my daughter brought home her new boyfriend. Lovely boy. Good looking, amusing, intelligent, beautiful manners, charming parents.

What luck, I thought, and what a relief. Not even a motor bike to worry about. The relationship thrived and eventually they married. I was very happy. My little girl wed to a man who was good enough for her. I loved him, and his parents loved her.

Now the happy-ever-after is over. Why is complex and not for parents to speculate about, but how should we all behave? Her parents-in-law always treated my daughter with love and generosity. They absorbed her into their family as closely as a child of their own, and she returned their warmth. Similarly, my son-in-law was as integral to my family as if he had been born into it.

Now that he has a new partner, my daughter finds that not only has her husband gone, but her extended family has as well. Her place at family gatherings is occupied by a stranger with no background of shared births and deaths and marriages, no memories or jokes in common.

It was usual years ago, and probably still is, for many deserted wives to feel the loss of married status keenly. This is about more than social position; it is a kind of amputation without anaesthetic, a widespread insult following the specific injury.

I feel for my daughter's in-laws. They are good and kind people. But now they must accept their erstwhile loved daughter-in-law's dismissal and embrace a stranger, or risk alienating their son.

For my part I am angry and deceived and sad. I could, seriously, wring his neck for hurting my child, but there is no sudden death of my affection for him. I miss him, but I have to stand by my girl.

We exchanged Christmas cards with his parents last year with stilted embarrassment. We have shared the same fabric of our lives for more than a decade. We have not quarrelled. We are just not able to communicate.

There is no protocol, you see. It would be so much easier if there were an established etiquette: who sees whom, and with whom, and when, and where. And should the wayward groom's parents now refund at least some of the enormous cost of the once-in-a-lifetime wedding?

The rules of coming together are so defined; the bride's family does this and that, the groom's family does the other. Coming apart is a mess. What is the Done Thing in these sorry circumstances?