Gf hanging alone with another guy?

Okay so my girlfriend has recently made plans to hangout with another guy, alone. I do trust her but I still think It's totally inappropriate. I brought it up to her and she freakkkked out and told me to shut the f*** up, yeah I know really shady. But eventually she calmed down and told me It's no different than her hanging with one if her gf's and insists that this guy is simply a friend. This is the first time It's happened but Idk how many more times it will because it will always scare me. I suggested that maybe we should just be friends because it appears this guy is just too important to her. She called me crying and bitched me out in essay format lol but anyways we agreed to keep the relationship going but I seriously don't know what to do?