When you say "video" we assume you are talking about the webcam for this particular Attendee, not a video file residing on that attendee's computer.
In order to have the audience see somone's webcam, that Attendee must be a Panelist or Organizer. You will have to promote this person to be a Panelist, and then have them share their webcam. If you don't want that person to speak, you can mute their microphone, and they will not be able to override your mute.
