Does she have to return the dish to the owners of the furnished apartment?

Firstly: 

Renting a
furnished apartment comes under the rulings on rental contracts which the
fuqaha’ have discussed in their books. 

This type of
contact is common and people engage in it on a daily, monthly and annual
basis, so it is worth learning the rulings on it, because there is no
transaction that takes place between people in any time or place but it is
subject to the rulings of Islamic sharee’ah and the guidelines of sharee’ah
which take care of people’s interests and seek to ward off harm. End quote
from al-Mulakhkhas al-Fiqhi (2/114). 

Among the
rulings on renting that the fuqaha’ have explained is the fact that it is
permissible for the renter to lend what he has rented, as is stated in
Mughni al-Muhtaaj (3/315), and (the borrower) may use it then return
it. 

Based on
this, there is no sin on you for lending this dish to your friend. 

Secondly: 

A rented
apartment or anything else that people rent is a trust in the hand of the
renter. What that means is that he does not have to pay compensation for
anything that gets damaged, so long as he did not transgress the limits and
was not negligent. 

It says in
al-Mawsoo’ah al-Fiqhiyyah (1/27): 

There is no
difference of scholarly opinion concerning the fact that the rented item is
a trust in the hands of the renter. If it is damaged without any
transgression of the agreed terms of use, or any failure to protect it or
guard it, then he does not have to pay any compensation. End quote. 

See
Badaa’i’ al-Sanaa’i’ (4/210) and al-Mughni (5/311). 

If the
rented item is lost due to negligence or transgression on the part of the
renter, then he must replace it if it can be replaced with something
similar, or pay its value if it cannot be replaced. 

It seems
from the question that the sister fell short in asking about the dish that
she lent to her friends. She should have asked for it from those who
borrowed it. As this did not happen, this is a case of negligence. In that
case, she has to compensate the owners of the furnished apartment for the
dish, either by following up with the friends who took the dish, and taking
it from them and giving it to the owners of the apartment, or by buying a
similar dish for them as a replacement. If there is no such dish available,
she should give them its value. 

And Allaah
knows best.