> I've tried to convince them of the general excellence of Mastodon, but without any success, so far.)
Mastodon is a Titter-a-like, and like Titter, is for old people (which is why we're here). But you might have a chance of convincing them to use something like a PixelFed service, or Loops.
You have an even better chance if you show them that they can create their *own* PixelFed or Loops service. Explaining that anyone with a browser can see what they post, as well as fedizens.