Is seeking separation from one's fiancée considered a religious obligation if she was referred to as "my darling" by a client and concealed this from her fiancé?

The fiancée is a stranger to you until the marriage contract is concluded. Therefore, it is not permissible to communicate with her without a valid need, nor should you delve into her private matters. Her response to the customer is inappropriate, but as long as it was a one-time occurrence and not her habit, do not dwell on it. If she is upright and of good character, do not annul the engagement. You can stipulate that she leaves her job before the marriage contract. However, if a flaw in her religiosity or character becomes apparent, then it is better to annul the engagement, as this is permissible for both men and women when necessary, to protect oneself from permanent harm.