Not difficult but it takes a bit of time as you’d take the Red Line to Union station and then Amtrak to BWI. You could do that to pick them up and then rent a car for the weekend at BWI for the weekend if you wanted one and return it when you drop them off (or, if feasible, let them return it so you don’t have to make the trip). Or let them take the train to Union Station if they can manage and you meet them there and then metro to Bethesda.