"It was so obvious that it makes it retarded that no one tried to check the motorcyclist. Really bad (lazy) writing on Fukumoto's part. Making your opponents retarded is not the way to go."
It's been pretty well-established that the motorcycle dude comes and goes every single day wearing his helmet, and on top of that, they had a much more suspicious-looking (from the lookout's perspective) person to be watching for. While the motorcycle dude was business as usual for them, he stood out more to us as outsiders. Additionally, the lookout didn't necessarily know that Kaiji and the motorcycle dude were friends, and thus they couldn't realistically expect them to cooperate. This is certain because the bad guys had to be stealthy and in a hurry, so they couldn't investigate peoples' relationships within the building, let alone their relationships with Kaiji. Overall I don't think it's that ridiculous: it seems obvious to us, but not to the characters in the story.
Don't get me wrong though this last bit has been kind of boring other than Kaiji's mom being cute and funny.