An enclosed litter box as well and scent-masking cat litter really helps. This in addition to cleaning the box daily of course.

We just keep our littler boxes clean all the time, it seems to work well and keeps the cat willing to use the box whenever it needs to go.

Tell your friend that kitty needs her/his litter changing every single day, without fail. Poor kitty should move homes if that's the care it seems to be 'enjoying'. If he/she is too darned busy to clean the tray, then DON'T HAVE A CAT!