I fing give up. The metal plate shown mounts to a 4.25x 4.25 welded metal box in the corners. These are not typically seen in residential or inwall environments. The device mounts to that metal plate and cover mounts to device. That backplate does not have holes to mount in a standard 2 gang in wall box. If he has the metal box inwall great. The cover with 6 holes might do the job again. If he has a standard 2 gang box. But who wants to see 6 screws.
Maybe we need a picture of existing box type