He never mentions Grainné in front of Medusa. Part of it is because he only keeps those memories of his past life preserved in a special corner of his soul. There were blood and tears but there were also joys and laughters. Whatever happened in his past is vivid proof of his life, his existence as a man named Diarmuid.
Also, part of it is because mentioning his past wife in his current lover’s presence is a very rude thing to do.
Medusa knows of Grainné anyway; anyone knows him should already be familiar with his legend. She is envious of Grainné and makes no attempts to hide her feeling. He could understand if she was jealous with Grainné but Medusa has made it clear that it is envy, not jealousy that resides in her heart. When he asks her for the reason, Medusa replies in a quite tone :”She was such a lucky woman. If I had ever met you, I would never have ended up a monster as I was.”
He does not know what to say to her tear-brimming eyes; instead, he embraces her tightly.