Exactly this happened among my acquaintance. The widower failed ( for whatever reason - could possibly claim incipient alzheimers ) to report the death of his wife. When he eventually died, the executors, who knew nothing about the "must-report-the-first-death" clause in the pension arrangements, informed the pension company, who then enquired about the first death ( twenty years before). They demanded and got repayment of the excess pension. Then the executors tried to reclaim the income tax which had been paid on the returned money. Unfortunately, there is a time limit on refunds of tax, so the family suffered the loss twice over, as it were.
In your case, the widow could possibly be charged with some offence, if the pension company could prove she should have known about the clause. Usually, these pension companies send out an annual letter saying "please confirm that Mr X and Mrs X are still alive". If she returned this letter claiming that Mr X was still alive, that looks like proof of fraud. However, the pension company would probably settle for the repayment, and probably not charge a very elderly widow with an offence.