So, the guy is his previous classmate/friend, but what is his issue with the MC? I hope it's not a petty reason, because the whole manga is pretty good so far.
Her mother was right there, and she made it seem that it was her mother that abused her. If he had gotten her to the police she might've been returned to her mother, and she might've gotten worse abuse.
Also, he was apprehensive in following her to the love hotel, but since the owner/staff seemed like he was hidding her, he trusted her.
Lastly, do remember that she made it seem that she was a run-away girl when he first met her at the market, and the ice cream store.
No matter what, you think its a good idea following a minor into a room in a love hotel eventhou the staff know about it? People can easily flip the story on him. They can hide else where inside that building eventhou that itself already a big risk, but following inside the room? That like hammering the nails into his own coffin. Trusting a stranger is the last thing he should do while being the target himself. Whatever you say, if the investigator girl didnt watch the camera view on her laptop, he fucked up bigtime. The mic on the camera really save his ass too.
Plastic surgery dude already doing alright, but went on being extra retarded in his end game of something related to mc. Dude just want to be mc or some shit? I hope its that and not some butt stuff.