Dear Mary: how do we deal with a neighbour who is stealing our friends?

Q. A dear friend of long-standing has a propensity to make friends of our friends — people she has first met in our house. She invites them to hers, some even to her principal abode nearly 650 miles distant, but doesn’t include us. Some have then invited her without us, some have even taken holidays with her. I don’t think we are paranoid, and perhaps we really aren’t that interesting, but still it’s not terribly pleasant, especially since some of these old friends, having been siphoned off by our neighbour, now have less time for us. Sometimes we vow we will not introduce her to any new friends but this seems rather petty — especially as we enjoy hosting small dinners and our friend is very convivial. We are sure she has no idea of the havoc she is wreaking on our feelings.
— X and Y, Saône-et-Loire

A. Give a small dinner to which you invite your convivial neighbour, one of the siphoned friends — and a garrulous plant. The plant should be scripted to pose of her fellow guests the perfectly natural question: ‘How did you two first meet?’ When they answer, the plant can gasp that she too has met wonderful people at your house and made friends of them. But how devastated she, the plant, was when she learned that friend-poaching was viewed as a breach of etiquette, unless you invite the person who introduced you to the first lunch or dinner with the new person.(True.) ‘Of course,’ she can continue pleasantly, ‘once I was told this, I realised how hurtful my behaviour had been in not including X and Y. And I apologised to X and Y and they were kind not to hold it against me. But I was so ashamed to be the only person who didn’t know about this social rule.