An Embarrassing Incident for the Arbitrator

Professor Grover Harrison has been jointly selected as impartial ad hoc arbitrator by a union and management, none of whose principals he has ever met. Eating his breakfast alone in a booth in the dining room of the hotel in which the hearing will shortly be held, he overhears the following words emanating from the next booth:

"Well, of course, it's not the truth, but if we're to have any chance of winning this thing. We'd damned well better consistently stick to our claim that the supervisor on at least on occasion made lewd and suggestive remarks to Mary. She can be counted on to testify this way at the hearing, and she's a good enough liar so that there's no chance of her being shaken in the cross-examination."

If you were Harrison, what (if anything) would you now do?