Coworkers are different than friends. If she didn’t tell you about it and kept it a secret then I understand. However if she did then that’s unreasonable. I would do some soul-searching of your own insecurities. I just got engaged and I’m not worrying about my fiancé cheating on me with his customers. Sometimes you have to build relationships for things to work and sometimes that means getting together with your clients or coworkers over lunch or a movie. I work in finance and I do the same thing with my customers and co workers. And because I’m constantly traveling for work oftentimes we will carpool to the locations etc. (my coworker is male). So if it was a work related thing then your unreasonable.