If the tenant has a problem with grease, they should should be using something like Bio-Clean
Within an hour after pouring the bacteria into the drain, the bacteria begin to eat their way into the waste that has accumulated on the sides and top of the drain pipe. This is their natural food. They digest the waste and spread throughout your system, cleaning it completely.
It sure beats snaking the line every month.
And like Joy mentioned above, grease does not belong in the drain.
I pour my grease in a can and let it cool, then it's in the trash, not the drain.