The play does not give a clear-cut answer to the question and this is typical of early Pinter's plays where characters are not always fully introduced and presented as we would expect in a realist play. The man Beth is reminiscing about in the play may be her husband, a lover other than him or simply a figment of her imagination and reverie. This man is clearly important in her life as she identifies him as "my true love" before the final curtain. Beth and the man had sexual intercourse on the beach and the woman has come to idealize that moment in the course of the years. Yet, this relationship is definitely something that belongs to the past and bears no positive influence on the character's present life. If the man was indeed the husband, there is no sign in the play that this is a shared memory which can contribute to restore harmony in the couple's marriage. Lost as she is in her remembrance/reverie, Beth completely ignores her husband.
Harold Pinter’s “Landscape” resembles the plays of Samuel Beckett in that the characters’s identities and relationships are only revealed to the audience through a glass darkly, as it were. The man Duff may be a husband or lover of Beth, but as the two characters seem unaware of each other and do not respond directly to each other all we know for certain is that Duff provides a male perspective on whatever relationship he was involved in (including revelation of infidelity) and Beth provides a female perspective. It may be that we are entering into the conversations of two characters who are so well acquainted that the backstory, infinitely familiar to them, never emerges explicitly, or it may be that the indistinctness of detail is meant to convey the universality rather than particularity of the characters’ experiences.
The man with whom Beth shared the sexual encounter on the beach may have been husband or lover. It may have been Duff or may have represented her cuckolding him or may have been something else. Structurally, it serves as a parallel to Duff’s adultery. Other than that, we really do not know.