Peeing in a sink or bathtub is almost always a sign of UTI or crystal formation. The fact is that these placs feel "cool" and therefore feel good if they are experienceing a bit of burning or pain. But the fact that it is combined with pooping may mean something else. I would still rule out the UTI/crystal formation, because the pooping could be just a "Habit" caused by doing the peeing in the sink. And since male cats can block so easily and it is a LIFE-THREATENING illness, please ask her to have it checked out.

Only after that cat is cleared of illness can you start thinking of other possible problems.