I wonder if the chaperone happened to say on the phone that, in checking around for Natalee, they had heard that she was last seen getting in that car with three guys.
That is possible. If Beth believed from the start that she was kidnapped, then any talk of her being seen going off with three guys would translate to "she was kidnapped by three guys". However, I believe Beth refused to think of Natalee being dead, so whatever was or wasn't said by the chaperone, I think Beth would think "kidnapping", not murder or manslaughter or negligent homicide or accidental death, etc.
Surely the chaperone and Natalees friends would have been discussing any chronology that was known at that time, and chaperone would have shared all of it with Beth.