I hunted for the regulation as if the poster is going to confront the electrician he must be certain it is against the rules.
521.10.1 Non-sheathed cables for fixed wiring shall be enclosed in conduit, ducting or trunking. This requirement does not apply to a protective conductor complying with Section 543. Non-sheathed cables are permitted if the cable trunking system provides at least the degree of protection IP4X or IPXXD. or if the cover can only be removed by means of a tool or a deliberate action.
The above is likely what you refer to however in my definitions it does not define sheathed looking else where we get "encase (something) in a close-fitting or protective covering. "her legs were sheathed in black stockings"" and one has to decide if the PVC which is around each wire is the sheath or if the insulation which binds the cables together is the sheath.