First and foremost, it's essential to respect personal boundaries and privacy in a relationship. Going through your partner's phone without permission is a breach of trust, and it's not a healthy way to address concerns or suspicions. Trust is the foundation of any strong relationship, and violating it can lead to serious problems.
If you do discover something that bothers you or raises concerns, it's crucial to communicate openly and honestly with your partner. Instead of accusing or confronting them right away, initiate a calm and respectful conversation. Express your feelings and thoughts about what you discovered, emphasizing that you went through their phone (which was wrong) and acknowledging the breach of trust from your side.
During the conversation, allow your partner to explain the situation from their perspective. There might be a reasonable explanation for the late-night hangout with the guy, such as a platonic friendship or a social event. However, it's important to be attentive to their response and how it makes you feel. Be open to discussing boundaries and expectations in your relationship to prevent similar situations in the future.
Remember, communication, understanding, and trust are essential components of a healthy relationship. If you find it difficult to handle the situation constructively on your own, consider seeking advice from a professional therapist or counselor who can help both of you navigate the issues and improve your relationship dynamics.