There's no explanation and it bothers me.
Part of me thinks Fujita was involved with that guy before becoming Kawasaki's stalker, because in before that we see him working in a bar and a man has his arm around him. And after he starts stalking Kawasaki he starts working in the cafe maybe he broke it off with the guy, which would explain why they were arguing.
http://www.mangago.zone/read-manga/oko_sama_box/mi/0a09806df74791cdd3f925457fd072d6_chapter-5/19/
http://www.mangago.zone/read-manga/oko_sama_box/mi/0a09806df74791cdd3f925457fd072d6_chapter-5/20/
But Fujita said it wasn't like that and I doubt he was seeing someone while stalking Kawasaki, and since he started stalking him once he got his address and Honda said he gave Fujita the address right after high school, I don't think there was much time for a relationship. But then again, I don't know. I needed that to be addressed.
Also, he was staring at his phone in a bed, with just his shirt one after that panel of the guy putting his arm around him.
I wish it was addressed too! It still haunts my soul, but I've asked numerous people from different translations and they all said it was left unaddressed. D:
So who is that guy who appeared at the end of chapter 3? What is his relationship with fujita? Up till the end, this part was never addressed.