I have some theories but nothing concrete. For a guy you might have to raise friendship with the guys passing out milkshakes for them to slip up and say something they shouldn't? And raising the BFF's lust stat may be a requirement in furthering the relationship with them, tho I can't confirm that. Try inviting the BFF on hangouts at the arcade or gym or to study while wearing skimpy clothing and not wearing underwear! Then if you're still waiting on the event when going to your dorm at night you can try just walking back and forth from the hallway to your dorm till it triggers, that's what I did 😂