[Miss Marple]

Excellent logic, livius, and a darn good question! There is even the possibility that the passenger was an accomplice.

By the way, why is she being "counseled?"

[RS_Rider]

I see this as happening at a regular bus stop. The killer knows that the bus will stop and be lighted inside. I imagine that the passenger has just been through a very awful experience thus the counseling.

[Catspaw]

I'd bet that the passenger is not being counseled but questioned: what'd you see, what'd you hear, where were you, etc. There may be counselor there, because if it was me, I'd be on the upper range of shock, but the first priority is to question the witness.

[.30Carbine]

By the way, why is she being "counseled?"

That was the wording of the eyewitness I heard on the radio, linked to from Imus in the Morning show. The eyewitness called a newsroom and reported the single female passenger on the bus. It was the eyewitness who said the woman was being "talked to or 'counseled' by the police." I would assume she was being both debriefed and comforted.