The deceased passenger is entitled to a refund per the international COC Rule 23(B)(1). This is crystal clear, and shame on the agent your friend spoke with for not knowing this and not bothering to find out. I mean that--this particular class of passengers who need something is likely facing the worst moment of their lives, and Delta should make sure that agents know what to do and how to do it if they get one of these calls.
Technically, all is says about the wife's ticket is that "Delta may waive certain fare restrictions in accordance with its written policy." My guess is you could wangle a refund from a sympathetic agent in this context.