"Now, she stopped working in 2018, so really doesn't have a qualified 401(k) for 2019"
That's not necessarily true. If the 401(k) plan uses a fiscal year instead of a calendar year, she is covered for 2019 because the plan year for which the contribution by elective deferral from her 2018 pay was made has an end date in 2019.
The marking of box 13 Retirement plan on her 2019 Form W-2 indicates whether or not she is considered covered for 2019. If you think the employer marked this box incorrectly, it will be necessary to take that up with the employer.
