If you have Beth at lvl 4 you need to study in the library to reach her lvl 5, if you have her at level 5 and own the special dress there should be a new option when [Talking] to her about a date, I hope this allows you to reach it.
If it doesn't I'll need more info because in the testing phase it worked.
If you mean the mystic dress we can buy from the shop in the mall? Certainly have that and lvl 5 Librarian. Talking to her doesn't seem to give me new options she just gives me the line about being busy. Doesn't matter the time of day or what my charm/might stats look like.
I suspect, at a guess, that I broke something by maxing out Librarian first but that's my only guess.
