It is indeed hers to solve, and no doubt difficult to watch from afar.
She is going to have to dig down deep and find courage and her ethical compaas, and get over being nice and what people will think. She is being used and taken advantage of with her full knowledge, and I am sure she is deeply affected by this, especially when they have ignored her requests.
I have listened to variations on this over the years from various members of our USNA sponsor family. Interestingly enough, it never seems to be the other way around, woman disrupting the man's dorm room - probably because the men would likely say, "hey, (nasty epithet of choice), cut it out" with no thought of what anyone would think of them. This couple is knowingly trading on the fact your DD is suffering from being too nice and worried about labels.
I don't know what the rules are for in-room visitation for dating couples.
Time for your DD to realize those high-flown words of leadership in her essays over the years also means taking a stand in social situations.
She should write down the facts of this issue, with when it started, what occurs, frequency, impact on her, actions she has taken, and response. She can either draw a line in the sand and inform her roommate she intends to file a documented complaint in 24 hours with the chain of command, or just do it. She is going to see iffy situations as a junior officer - it will be on her to act. This is a step toward developing what is called "command presence."
I can say from experience that action usually feels way better than reaction.
I also understand the social pressure to not be "that girl," the complainer, the ratter-putter. There is a limit, when the personal impact outweighs the social urge to just play along.
One of our USNA sponsor daughters decided to respond in a parallel way. She enlisted a group of supportive friends whom she texted everytime roomie had the BF in. Two or three would arrive, study materials in hand. They would turn on the lights, talki in normal tones, play music, get up and move around, open and close the room door - ignoring the couple completely. In a hilarious moment, one of the friends wandered over to the couple on the rack and just started a conversation about a mutual class. The behavior stopped after about three nights. If roomie could have her bf in and not care about impact on others, Righteous Roomie could follow room visitation rules and have her own friends in.