I'll bet Jill would attend if a faithful Mc asked her to. Her identity would be hidden by her costume and it is a chance for her to expand her horizons (and she seems like the type who would enjoy a little costume dress up). Anyway, the girl is 21 years old and has never had any college fun ffs. It's time to attend a party. She wants to compete with the other girls and squeezing that 10/10 body into a sexy little outfit would definitely turn some heads.
MC has already invited her to see his room, so now is the perfect opportunity. If she is uncomfortable, they could slip away to his room for private movie, guitar playing, and who knows...maybe a boob will be slightly grazed.