If the widow doesn't know how much income her husband had, it's not a good sign that she wants to rely on what IRS knows because that might not be complete.  I have seen too many cases of malpractice where the knee-jerk reaction is to file a joint return, even with a balance due.  Figure the tax separately, and if the decedent owes taxes that have to be paid from the estate, figure out how much the widow would lose.  In many cases she has priority even over tax claims.