Contractors working on property owned by others have a fundamental duty to preserve and protect that property on behalf of the owners. Anything found on the property is owned by the property owners, period. The contractor was allowed entry to the property for the purposes spelled out in the contractual agreement, and no other purposes. Taking the revolver was a theft, and since the theft occurred within a dwelling there might also be a charge of burglary (depending on state laws; here in Colorado the law specifies that entering into or remaining upon the property of another for the purpose of committing a crime other than trespass can be construed as burglary).

Regardless, the contractor is certainly exposed to possibly serious criminal charges, and since he has told at least one other person about the incident it will only become increasingly likely that everything will come to light.

Best recommendation I can offer is for the contractor to explain everything to the property owner and return the revolver to that owner, then cooperate fully with the property owner's wishes on any reporting to be done to authorities.