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.