Are you sure you are not confusing with our introduction to them to when they first meet? Because Rinea has no reason to go to Zofia castle as she wasn't a soldier or a major Rigel noble. In fact she hated war. Another reason is that Zofia Castle was barely in Rigel's possession before it was taken back in less than a month and they seemed to know each other a lot longer.
In fact here is what the wiki states:
"Before the events of the game, as recounted in a memory prism, the two met at a Rigellian ball. Rinea, having retreated to a courtyard away from the main event, is discovered by Berkut and invited by him to dance. Berkut, noticing Rinea's shyness in dancing around the other nobles, agrees to dancing at their current location away from the festivities."
I don't think the main ballroom would be away from the festivities...