They can send the bill. Often it seems to take the insurance carriers and secondary insurance carriers approximately that long to pay their share of the claims. There is a statute of limitations, veries from state to state. I received several bills of this type two years after my husband passed away, some of what was included were invoices on my children from three years prior. I contacted the insurance carriers who had to go back through records from years ago to find out if they had even been billed by the physicians/hospitals/etc. for payment. Several times they had not been invoiced for the procedures. The allotted time for billing the insurance companies had long before lapsed. You may find the primary carrier dallied so long in paying their share of the bill the secondary was not billed in the allotted time frame. You can argue the bills. Most times they have all ready been written off by the facility anyway for tax purposes and they are just hounding for additonal payment.
My next question becomes, was the estate settled? If so then the notice was published in the papers, etc., and if the bills were not received within a few weeks or months (check with an attorney for the actual time frame) then while they can attempt to bill her she is not legally responsible for the bill.