"Her cats eat the mouse."
Translation:I suoi gatti mangiano il topo.
It depends whether the cats are male or female. In English "her" does not tell us the gender or number of the cats. In Italian, the possessive takes its gender and number from the noun it describes instead of the owner. The Italian "sue", for feminine cats "gate", could have the owner which is "lei" (she) "lui" (he) and if capitalized "Sue" is for "Lei" (formal singular version of you). The Italian "suoi" is for masculine cats "gatti" regardless of the owner.