Abate understatement penalty

Joint return?  What is the widow's excuse for overlooking it? She didn't know about the income, just signed the return like she always did?  It's unfortunate that she paid it first and now wants to question it afterwards.  You should check that the supervisory approval requirements were met.  This sounds like a CP-2000 case where IRS will argue it was untouched by human hands -- but will admit that every return is reviewed by human eyes. I had one case where the supervisor just checked a box but did not indicate approval. Settled it on other grounds.  

Via the Form 843, explain and request that the penalty be abated for reasonable cause.

Assuming that the IRS deems the cause to be insufficient, ask that the penalty be abated under the IRS' "First-Time Penalty Abatement" procedures.

Hopefully the forgiveness for cause will work so that the client can keep the "First-Time Penalty Abatement" wild card in her pocket for future use. (The IRS wipes the slate clean after 3 years, so the First-Time Penalty Abatement resets and becomes available again, given time.)