All borrowers on a mortgage are jointly and severally liable.
That means that if one does not pay, the other has to pay the full amount. They are not liable for just half the balance.
I assume that the executor of the estate should discharge the mortgage in full from the estate before paying anything out to anyone else. Of course, he will be able to do some sort of arrangement with the owner of the house and with the bank. The bank might allow the borrower to take over the full mortgage if they have the repayment capacity.
Brendan