Shakily, he begins. "Sir, what can I say. We came here to be advised. We sat in the waiting room, we waited. Mr. Bennett never actually opened his door, which frankly we found surprising."
"And...?" You ask, hoping to spark something.
"And what?" Mr. O'Brien exclaims. "We waited and waited and the girl ran out screaming. That's all I know."
"Sir, can anyone prove that you sat and waited?"
"N-no sir. At least, I don't think so. Everyone was rather groggy this morning."
What? A full waiting room and nobody can prove it? The problem isn't who saw O'Brien. The problem is why didn't O'Brien see anybody else who could confirm the alibi, including the detective himself?
I'm going to guess that they weren't actually in the waiting room the whole time, if at all. They showed up early, killed the victim before their appointment, then dropped in for the actual appointment later to try to deflect suspicion and saw the receptionist run out screaming.