I guess. You make some good points, given who Rika is and that she's also friends with Akira, and that she has messed their relationship once already.
But, again, if it were me there'd be no chance in hell I wouldn't scroll through the entire cellphone of a 30 year old dude that my 15 year old daughter sent a masturbation video to. Especially given that all girls seem to be smitten with him, and that he's their teacher.
It's way too suspicious already, and way too easy to simply look when you're already in their cellphone - there'd be literally no repercussion, he wouldn't be able to tell if she saw one or all of his messages.
Two things:
One, i think you're not accounting for your bias enough, you
know what Akira is, does she? If it hasn't even occurred to her to suspect something, she is an airhead after all, then wouldn't it be an illogical leap for her to go from 'my daughter is hitting on this guy' to 'and he's probably fucking his students!'... seems like an absurd leap to me, sure, some people might make it, but Rika?
Two, what did she do? She read it, freaked, and went straight for Rin, she wasn't thinking of Akira, she was in a panic and completely tunnel-visioned on her daughter... maybe she'd have thought to do it after, but i've been in situations aplenty where you get so hyperfocused on one thing that your eyes may as well be pinhole cameras...
Edit: actually, a third... i don't think you're accounting for the fact that some people really do just respect each others privacy, either due to disinterest or honor. I can understand the urge to snoop, but i've long had the impression that most people feel that urge far
far stronger than i ever have.