Should I stop my fiance from going to dinner with her boss?
Why don't you like her boss?
That stupid bitch is a slut, I know she wants to sleep with him.
I'm sure that isn't the case. I don't think your should prevent your fiance from having dinner with their boss, because it might come across as being controlling.
I know relationships are about trust but I can't trust her for this.
If she hasn't given you a reason to not trust her, then I don't think there's any issue with her going to dinner with her boss. I think you shouldn't try to prevent them from having dinner, and if you still have reservations about trusting her, then the two of you should sit down and discuss the matter.