Prostitute, whore, potato pohtahtoh, Anyway It triggers randomly afaik, you are probably too far into her relationship to trigger anything. You could try talking to her to trigger it since it should technically work after she leaves the Halloween party early (you will know when all characters are there but she isn't) then at some point she drags you to her room and starts a conversation with something like this:
View attachment 5032208
Gameplay-wise is the best way to start the relationship since you get some easy points. Tho I don't like doing it.