And "Show, don't tell" is just a rule. Whilst rules must be known, they need to be declined to each case, and this case doesn't require a showing of Yuzumori and Ririha's past together.
About the "show, don't tell" rule... I think there is some good exceptions to this rule... For instance, the movie Halloween 1978 (weird choice but I pick it anyway), there is this speech by Dr. Loomis about Michael Myers:
"I met him, fifteen years ago. I was told there was nothing left; no reason, no conscience, no understanding; and even the most rudimentary sense of life or death, of good or evil, right or wrong. I met this six-year-old child with this blank, pale, emotionless face, and the blackest eyes... the devil's eyes. I spent eight years trying to reach him, and then another seven trying to keep him locked up because I realized that what was living behind that boy's eyes was purely and simply... evil."
(this speech is without a flashback btw)
Michael Myers didn't fully appeared in the screen at the beginning of the movie, the mistery of how this phyco is, and what he can do, is what build up the tension and the atmosphere of the movie. And this speech only reinforces the tention.
Why I said all that? I just think that the "show, don't tell" rule isn't a horrible sin that everyone says to be. It's just my opinion, don't hurt me, I'm a good girl.
Is this rule good in Yuzumori-san? Well... What would be the mystery of why Ririha does what she does if we already know what it is?