I find Duke unusual in that it requires students to live on campus through their junior year. Were seniors living on campus when you were there? My student lives on campus as a senior because it is convenient. And I was surprised to find that many in the circle of friends, also seniors, were staying on campus also. I do recall the room next to student's was empty in her soph year. The resident had to pay for the room because s/he wasn't a senior; but actually lived in an apt. off-campus. I know that Duke is unusual in that it requires residency for 3 years; but is it unusual in trying to control drinking on campus?
My recollection is that seniors who could get single rooms stayed on campus, and those that could not lived off campus. It was so much more convenient to live on campus, and the homes the seniors rented off campus were pretty dumpy, just like 610 N. Buchanan. The drinking age was 18 for beer and wine (we came in just under the wire!) so there was not much effort to curb on campus drinking. That changed greatly when the drinking age increased to 21 -- the university has much greater liability for underage drinking and so has to crack down. I know Georgetown is also frequently cracking down on its campus partying, and the tensions between Georgetown seniors living in the neighborhood and local residents is legendary. So, Duke is not unusual.
By the way, the 3 year residency requirement is a cash cow for the university. Your student's experience is consistent with other anecdotes I have heard about empty dorm rooms because the campus is now so boring that social life has by and large relocated off campus. That means that the University is losing a source of money and one of the reasons I believe Duke turned a blind eye to Durham's policy of harassing off campus students.
Seniors lived on campus when i was at Duke, I lived on central.