Miss Manners: When friend of a friend quietly picks up dinner tab, how many thank you notes should he get?

Dear Miss Manners: My wife and I were hosting dinner for an old friend at a local restaurant. On arrival we saw another mutual friend, who also knew our guest. Those two talked for about 10 minutes while my wife and I were settled at the table. We had no qualms about that, since they had not seen each other in a long time.