This is one of the reasons I don't think it's a good idea to wear letters before initiation. Anyway, I would suspect that the sorority would ask her not to wear the letters, but they couldn't take them away. Her best option would be to sell the letters to a sorority member. I think if a member bought the letters, it would show a great deal of maturity (hey, you didn't join but we appreciate that you had the courtesy to want to return the letters).
