How should a divorced spouse handle IRS notice that indicates that the couple owe IRS $690 for 2016 taxes due to ridesharing carryover that was disallowed by IRS?

In November 2016, the married woman separated from her husband and filed for divorce. Husband and wife are legally divorced now since May of 2018. The ex-husband filed the 2016 taxes as MFJ without the wife's consent even though she was no longer leaving in the house. The wife is not aware whether the ex-husband received a refund or not.

Now, according to the IRS letter, a ridesharing carryover has been disallowed resulting in the couple owing IRS $690.

Question

How should the wife handle this situation since she left the house in Nov 2016 and did not participate in the filing of the 2016 returns?. She has no knowledge of what was filed and the outcome of the returns.

Your professional advise would be much appreciated. 

Thank you.