The estate, if it has the money to do so, would normally be responsible to pay her back for the rent. However, her claim against the estate would come after any tax liabilities or secured creditors and she would only get paid, if there is enough money left in the estate to do so.
Her position could be strengthened if she and her boyfriend had signed a written agreement about the amount of rent each of them was responsible for and what to do in the event that one of them passed away during the term of the lease.
An estate planning attorney can guide you to the answers to any similar questions.
