OK, then as long as she would prefer McKelvey to those public engineering colleges, and you can comfortably afford it, I see no harm in ED II at WUSTL.
But I would caution to have modest expectations about what ED II alone will do for her to persuade WUSTL to compromise on its normal numeric standards. So I wonder if there are other ways to show in her application she is actually really interested in Humanities and Social Sciences. This could be something done in activity selection, essays, and so on.