So, the couple closed their joint savings account and sent the £125k therein to his Lloyd's account (presumably because that was the nominated account for payments from the savings account), he wrote her a cheque for the full amount, she paid it into her Barclays account, they requested the wrong amount from Lloyd's and she didn't notice.
When the error was noticed, Barclays requested the money, but he had by now died, so the account was either frozen or had been emptied by the executor.
By today, the estate must have have had a grant of probate, and the assets disbursed. The cheque is evidence of a debt against the estate, but she will be out of time to request payment.
If he had died while the money was still in the joint account it would all have passed to her outside the estate, under the survivorship rule. The executor may well have realised that the money in the Lloyd's account had come from an account jointly owned with her, but chose to interpret the transfer as, effectively, a gift from her to her soon to be deceased partner, which seems hardly ethical, especially if the executor is a beneficiary of the estate.
A law suit would cost more than the value of the case, so her money is gone.
It may be possible to complain about the executor, but I don't know how, once an estate has been wound up, and its probably expensive as well.