What did Mary and her friends discuss at dinner while staying at Lord Byron's house in Switzerland?

They all discussed to try their hands on supernatural stories.

Explanation:

Mary Shelly, Persy Bysshe Shelley, Mary's step sister Claire Clairmont, Lord Byron, and Dr. John Polidori were friends. In the summer of 1816, they decided to spend their time in Switzerland. They all stayed in Villa Diodati, the house of Lord Byron in Switzerland.

On one night, they all gathered together and were reading the German ghost stories 'Fantasmagoriana.' They all came up with an idea of trying their hands on supernatural stories.

It is during this time, Mary's most celebrated novel 'Frankenstein' was born. In the preface of Frankenstein, we come to know about the friendships between these prominent writers as well as the plan of writing the ghost stories.

So, they all discussed to write ghost stories at dinner at Byron's house in Switzerland.