But Delaware doesn't get to write tax laws for Maryland.  So I think you have DE source income under MD law, MD source income under DE law and (inevitably) DE tax on the worldwide income of its resident for the second half of the year (which would be eligible for a credit for taxes paid to MD but MD isn't taxing the income so that will be $0.)

If the employer didn't change withholding from MD to DE then have the client pull some paystubs for the relevant pay period and split the income between MD residency and DE residency.  If they do go into the office for meetings, etc. there could be some MD source income after moving to DE but those are facts not in evidence here.

Welcome to the multi-state tax party (and why I got rid of almost all of my non-VA clients when I scaled back a few years ago!)

Rick