It could well be WORSE than flushing sand down.
The products nowadays are oftentimes designed to clump
when they get wet. And others are clay dirt that will do
that to some extent. So you'll tend to get a mounding and
blocking effect in the pipes, especially if they are the old style
clay pipe in the horizontal run.
But I'll bet they have PVC pipes, which shouldn't be quite as
vulnerable.
The other example, (gravel) would indeed be bad, worse
than sand or cat litter, but also pretty unlikely.
Tenant needs to be reminded of the type solids that are to
be flushed, or not flushed. And that the inconvenience
encountered because of the litter flushing habit will be translated
to his need to be searching the Yellow Pages for a Ryder truck
when the landlord stops accepting his rent.
Most people have e-mail nowadays. Rather than trying to forbid,
just forward these comments to the tenants, all the tenants that have
cats. Or establish a no-pets policy in the lease.

I-zheet M'drurz wrote in message
...
Flushing the actual cat crap down the drain shouldn't be a
problem (no more than larger human stuff) but I would *never*
flush litter down the toilet. You have it figured right, it is
likely clumping up in at least one place. It would be the same
as flushing sand or gravel, IMO.