Both of them have made mistakes and need to communicate better. Though if it were me, I would cope better with my gf saying "I'm going to be away for a week and might not be able to talk to you" than with "I'm going to a dating event with a person I just met."
Hopefully Kase realises how being away from her and living with another girl makes Yamada feel, and hopefully Yamada realises how being away from her and going to a date night makes Kase feel. Then they hug and kiss.