While Guy de Maupassant certainly could have provided a clear statement regarding Madame Lantin's infidelity in "The Jewelry," it does not necessarily follow that the inclusion of such clarification would have actually improved the story in question. Remember, every decision an author makes while creating a work of fiction is subject to various trade-offs, and Maupassant would not have been an exception.

With that in mind, it may first be worth clarifying that Maupassant (for all that his short stories often revel in ironic turns of fortune) tended to be very much a realist, and, from this perspective, Maupassant's decision-making here does make a certain amount of sense. After all, in real life we are not given all the answers but must grapple with various layers of uncertainty. Thus, the story's ambiguity might actually heighten its sense of realism, mirroring as it does the ambiguities common to lived experience.

Additionally, it might also be argued that this sense of ambiguity actually heightens Lantin's own angst regarding the discovery of the gems. After all, a clear answer on this question (perhaps via the discovery of his wife's correspondence, for example) would provide, if nothing else, a sense of closure. But Maupassant presents a much messier situation than that, one by which Lantin only has suspicions but is not provided a definitive answer one way or the other.