In the steps of a dead man (1974)

When a young soldier suddenly dies, his parents and his girlfriend are naturally shocked and upset by the news. Several months later a young man turns up at the parent's house claiming to be their dead son's best friend. They welcome him and invite him to stay for a while. They also introduce him to their son's girlfriend and she is similarly impressed by him. It is later discovered, however, that he is not what he appears to be and may have had a hand in their son's death.