Video preview continually ON

I am a 77 year old host who does not know too much about zoom. However we have used its a lot both for a Bible Study group while in lockdown and for family chats.

In the study group one of my attendees who is 76 has problems coming in.

When I send him an email with the meeting ID and password he is the only one who gets a screen "video preview' and he can do nothing after that.

Any help would be most welcome.