(wa State) Mother Died Years Ago, Creditors Are Now Coming After Dad

Back in 2021, my mother unexpectedly passed away without a will. Per the recommendation of my uncle(an attorney), I did not open probate(as she had no property or anything I wished to inherit), was named the executor of the estate to handle her affairs, and submitted a non-probate notice to creditors, which I was told that creditors could not come after her estate after 4 months.

Fast forward to several months ago, my father(her husband, 100% VA disabled and living in a nursing home) received a notice from a debt collection agency trying to pay for her hospital bills from a visit back in 2020. He has now spent thousands of dollars, driving himself completely broke in paying these off. I'm trying to find a solid answer as to whether or not he actually has to pay these debts or if there is anything the credit collection agency can do.