There are flushable litters on the market but I'm cynical about them myself.
Sometimes I'll scoop and toss THAT into the toilet, but in those cases there
is little litter on fecal matter; clumps of urine still go into a bag.

You'll need to get with your tenant on the issue -- confirm that he/she is
even flushing litter. It may turn out that this is not the case at all. If
it is, I'd definitely make notice that this is not allowed.

Have you tried snaking the drains, see if anything comes back out? I know
Roto Rooter is now offering a camera inspection of drain systems -- that
might allow you to identify the problem with greater certainty.

James