It all boils down to the WC's T&C's. Most will have a clause covering just this.
No law has been broken, it was no more than a conversation, a deniable one at that. Even if he did clean the windows it is still not a legal issue.
At this point all the employer could do if he heard such a thing is open up a formal investigation. Realistically, if he does find out this just needs to be dealt with properly, in line with the company's disciplinary procedure.