I get the feeling the ghost girl is noting how well she can control the zombie girl's body for less than pure motives. Well, I suppose that will make her have a better part in chapter 3!
So, the way to prevent a zombie's flesh from inevitably deteriorating is to engage in lots of sexy time, inclusive of (if the zombie is a female) her being (figuratively speaking) filled up regularly with semen.
EDIT: Somehow I think we all can deduce what would be the likely procedure to follow if the genders were reversed (male zombie and female normal human).
*crickets chirping*
Perhaps Only In the H-Universe, but ..... "I'LL ALLOW IT!"
By the way: Don†˜t forget to praise the Goddess of Sequels!