I gotta admit though, I always HATED when people talked on their cellphones in the hall during sleeping hours, I'm a very light sleeper and as we all know dorm doors are notoriously thin so I could hear EVERYTHING, so I think in this instance I would ratehr she be in the bathroom then in the hall. My next door neighbor who I ended up REALLY disliking was notorious for sitting infront of my door and talking about mundane things till 4 AM.
But I'm also thinking theres probably a better alternative to both the bathroom, her room and the hall - its called go outside, or go to the lobby as long as it doens't have doors to peoples rooms attached to it.