Dating the BFF isn't just about raising their lust, they're a special character so there are storylines to romancing them. Right now there are two options: the quad party line, and the dorm room event line.
Quad Party: Keep going to the quad party on Hanna South Road every Friday. Eventually you'll be prompted to investigate the milkshakes the boys keep giving out (if you're also a boy one of them will say something suspicious, if you're a girl you have to drink them and notice a strange flavor). Then go in and out of the building till there's an option to investigate a strange noise. Do that and get a scene with the BFF.
Dorm Room Events: Take a shower in the morning, and you may randomly get an event with the BFF. After you get it wait about a week and you can randomly get another event when going from the hallway to your dorm room. Choose to help them and get another scene.
After EITHER of these storylines (you don't have to do both) keep visiting them in the evening to talk about sex. Things will escalate until you and BFF have the "what are we?" talk, and you can choose to date them.