I guess I don't really know how the Japanese would feel about going to a mixer when you're in a relationship already. For me, I would only be worried if my partner didn't come back on the night they went.
Of course telling my partner and having them tell me is the most important part of that though. Maybe it's a huge deal in Japan to go to a mixer when you have a partner already and it's grounds for breaking up.
It just seems really stupid that Aoki didn't tell her that he was going to a mixer, especially right after they just had a heart to heart about their relationship. Pretty shitty of him.