What should I do if I meet a roommate who habitually doesn't go to the toilet?

I think I can tell him if I don't flush in the toilet, but it depends. If he is sensitive, then you accidentally touch his sensitive nerves, which is somewhat unpleasant. But I think this kind of person should change his attitude when dealing with him, don't joke, don't say in front of everyone that he doesn't flush the toilet, just mention it occasionally in private to see his reaction. Next time he doesn't flush, he will have an excuse to tease him (why don't you flush the toilet again). If he is the kind of person with an open mind, the subject will probably have communicated with him long ago, and no one will care too much about such a trivial matter.