As you indicated, it is not really possible to determine if the IRS will catch this, but if they do the consequences are not that bad. The RMD would be taxable as it currently is, plus there would be an excess contribution to her IRA that would have to be returned with earnings. Once the due date for her return passes and the IRS takes no action, but catches this later on, instead of the excess removal including earnings, there would be a 6% annual excise tax for each year that the disallowed rollover remained in her IRA. It is not likely that this would trigger an audit involving the entire return, rather it would just require an excess contribution correction.
Perhaps some of former PLR 1099R and 5498 matched up, but not others where the rollover might have been made to the surviving spouse’s IRA.