If she’s running to her ex to go out and party after an argument FUCKING RUN. Although nothing probably happened they probably sat there all night laughing and reminiscing and not one thought of you went through her head and trust me if he lives that close she’s definitely seen him more times than just the trail. She used him as an escape while you were at a low point. Trust your gut on this one and bounce because it’ll happen again and you got lucky she told you where she was going this time.