To my knowledge the official WashU policy is that ALL of their regular admission, full-time undergrad students must live on campus for the first year. See here:
All full-time degree-seeking undergraduate students are required to live in the residence halls their first year. This requirement does not apply to incoming transfer or incoming exchange students.
There is actually also a clause to that effect in the housing contract–see II.A. here:
However, does that mean there are never exceptions? I don’t know, but in my experience with universities there are almost always SOME exceptions to policies like this, for a variety of special cases they just don’t want to put in some public policy statement.
So I agree if this is important to you, you should work through your mom and her HR reps to see if you could get an exception. No promises, though, as to whether that would work.
Edit: By the way, being on the South 40 is a big part of the first year experience at WashU, and really a significant part of the general pro of WashU having a very good housing and dining reputation. Whether you can afford that luxury (if it will cost you) is a different question, but I do think part of the logic of the requirement is it gives their students this common experience that really is a big part of building their student community.