First thing is to rule out a Urinary Tract Infection. Once the cat gets a clean bill of health, the rule of thumb is one litterbox per cat. I suggest your friend go out and buy 2 more litterboxes so they each have their own.

It COULD also be that the litterbox is not clean enough for the cat. I check my boxes twice a day, having 8 cats of my own. Once in the morning before I go to work and once when I get home. Because it's summer and hot out, I absolutely HATE coming home and smelling my litterboxes.

Please keep us posted.