USNA is different today than when your girlfriend's father or I attended. At the time, although there were programs to help you succeed academically -- it was somewhat of a sink or swim approach. Today, there is greater recognition that even really smart kids may struggle academically at USNA for any number of reasons (see below) and there is greater emphasis on getting you help early and often. Across the board (academics, athletics, conduct), the view is that, if you really want to be at USNA, they will ensure you succeed -- but you have to put out the effort to do so, they won't carry you along.
That is not to say that, occasionally, USNA makes a mistake and admits someone who really can't hack it academically. But those cases are very rare. By and large, you get in, you CAN do it academically, you must want to put in the work.
People struggle for a number of reasons. First, many kids didn't have to do a lot of homework or work that hard in h.s. USNA classes are hard and, if you don't keep up with the work, you will start falling behind and it's really hard to catch up. Second, there is so much else going on other than academics and the pain for failing in those (chow calls, come arounds, room inspections, etc.) is more immediate than the pain of failing academically. Third, you take a lot of hours and there are few/no easy classes. Fourth, you must graduate in 4 yrs and must pass certain courses to graduate. And lots more reasons.
I think the biggest shock to most mids is that they really do earn Cs, Ds, and even Fs -- this for people many of whom have never earned any grade less than B. Their 3.89 high school GPA turns into a 2.32 USNA GPA. No one thinks it will happen to him/her but, after the first semester, one out of every two mids stands in the bottom half of the class and most of those are earning less than a 2.5. That is a rude awakening.