If a control system is out of the question i would go with two different model TVs.

TV #1 would be wired so that cable box audio is routed through the whole house system.

TV #2 would not be connected to the whole house audio system and it would always have the volume set to zero so that you can only see video.

The client would always use TV #1 for "listening and watch" and TV #2 for "watch".

IMO this is the least expensive least confusing way (for the client to operate) to do this.