She assumed that Miyuki wasn't, you know, a scumbag. A person she's known for about 10 years wouldn't be cheating on her best friend since preschool so blatantly and openly.
I think 10 years of being acquainted would give them some leeway and trust.
It's not like she was going to be 'Why did you break up with my son?', or ask her son 'Why did you break up with Miyuki?'.
It is literally stated in the chapter, meaning that your media literacy is really poor.
Trust explains why she didn’t immediately assume Miyuki was a cheating scumbag, it doesn’t explain why she refused to verify anything. Seeing your son’s girlfriend openly holding hands with another guy and going “welp, they must’ve broken up” without asking a single neutral question it’s just sticking your head in the sand.
And let’s not pretend the only possible questions were some dramatic courtroom interrogation. “Are you two still together?” exists. It’s free. It doesn’t accuse anyone of anything.
Yes, it’s stated in the chapter. Congratulations, we can read. Explaining her thought process doesn’t magically turn dumb assumptions into good judgment. Media literacy isn’t just repeating what the text says, it’s recognizing when the text shows a character making a mistake.