Tbh, that’s really weird and unnecessary. He wasn’t just an ex who she strictly hung out with, but someone who she was sleeping with prior to you. There’s literally no point in going out with an ex as a ‘goodbye’ lmao. As a 21f myself, I can see through her intentions. It’s probably hard for her to let him go, they probably fucked. But at the end of the day, it comes down to how much you trust her