Bertie is a non-canonical character created the episode The Red Signal of The Agatha Christie Hour, a TV film adaptation of the short story by the same name. When visiting a medium, Marion Partridge came in contact twice according to Violet Eversleigh. Jack Trent felt sorry for Bertie, for not being left alone by Marion even in death.
Bertie is not seen, just mentioned. It is never made clear if Bertie was the deceased husband of Mrs Partridge, or if he was another man that she pursued.