I Def agree generally speaking but i think this situation is a bit peculiar because all she saw was a guy going into the girls bathroom and then being lied to about why. I don't think she even "looked into it" per se unless i misread? I think she just started putting two and two together after she thought about everything. But what you said isn't wrong!
i agree the clues just fall to her lap and she just made the connection. she isnt trying to investigate him other than making sure he isnt sneaking in into the female bathroom unauthorized.
Fr, and Arisa seemed smart, it practically was just common sense for her
“she has a good heart she’s gonna help him” sure thats sweet of her but dont transvestigate someone oh my god even if you are the worlds best ally and you love trans people dont go out of your way to find out something that might be a secret. not hating on her or anyone in particular