YBR-- Stay out of the circle

Question…What are the rules of dating in the circle? Here’s the situation. Diane and Rob had been dating for about 9 months and decided to take a break. They weren’t dating other people, they were kinda in the 50% single category (read YBR 34). So a few weeks after the break Diane finds out that Rob went out with Susan. Susan is a member of the overall circle of friends they hang in. She’s at all the summer barbecues, holiday functions, girl’s night out. Diane and Susan have hung out before, so they are associates. Susan knows all about Diane and Rob’s relationship and break. Apparently, one night Rob calls Susan and suggests they just go grab something to eat just to talk and catch up. Susan accepts. Diane finds out about this quasi date through the circle of friends and is of course pissed. So I’m going to leave my opinion out of this one and just pose a few questions. Does Diane have any reason to be upset? If so who should be mad at, Rob, Susan, or both? What if anything should Diane do? Would Diane be wrong for now feeling uncomfortable with Rob and Susan?