This is probably the worst case scenario playing out. Even if they were able to bring Teru back to life and remove his memories of committing suicide his mum has still abandoned him. He's not going to trust the class rep MC anytime soon. All he'd have left would be Karagaki once she comes back, and let's be real she isn't a particularly good person to be Teru's last pillar of support.
Well...damn... I knew his home life was bad but, didn't expect it was so bad he would off himself, I guess that's why he was so desperate to fine karagaki and with her gone and his piece of shit mother abandoning him, that letter was the last little bit for him. Damn..