Net Work Book Club discussion

In my mind it's a book by Mary Stewart but I've been looking online and none of her titles seems to be it. I read the book in the mid 70s though it might be older. A woman and her husband (his name is Roarke) live in Italy? Greece? He used to be involved with an opera singer whose last name is Mallory, and then married his present wife, who is younger. The opera singeri is referred to as "la Mallory." Circumstances make the wife think that he may have started up the old affair again. The motif of a boy riding a dolphin (frieze, fresco, sculpture, something of that sort) is important.

Anybody remember it?

Not a clue, I am a horror and fantasy reader for the most part. Good luck on finding the title would help if I could but am truly clueless...