If the taxpayer wants a refund from MD, then it sounds like they never filed the proper form with their employer, and as a result they had MD tax withheld. My understanding is that in that case, they must file 2 returns and can't apply the reciprocity agreement for that year.
They need to file the form and make it effective for January 1, 2018, and file 2 state returns for 2016 and 2017.
