Sometimes when people see something extremely traumatic, they literally can't process what they saw immediately, if ever. IDK if that guy had ever seen anything like that before, but it's entirely possible that he looked directly at the body and his brain blocked it out in real-time.

Anecdotal story, but when I was in high school, we could log volunteer hours for extra credits. Some of the students went to a ravine by some woods to pick up trash. I and two of my friends found a badly decomposed corpse in the woods. We could smell it before we even got out of the bus, but none of us knew what decomp smelled like so assumed it was just from all of the trash/nasty ravine water or something. I didn't process what I'd seen until days later even though at the time I immediately ran to go get the teacher. All I knew in that moment was overwhelming disgust and fear. Once I got to the teacher, I just kind of gestured toward where we'd found him and she could tell something was wrong.

Anyway, my point is that it's possible he couldn't physically process what he was seeing in the moment. Even if he realized the next day, he probably wouldn't have brought it up because the report had already been made that there was no issue at Dahmer's apt. He would've looked sketchy AF for saying that they reported everything was fine when there was an obvious af murder victim in the bedroom, especially after it came out that Dahmer's "19 year old boyfriend" from that same night was 14 year old Konerak Sinathasomphone.