@dr_harlequin: I wasn't very bothered by the "I don't have many friends." line. But I suppose that's really a matter of opinion. I thought it added nicely to her interactions with the Joker and made a bit more sense of why she was falling for him.
Also, I don't think it took just one session to drive Harley insane. Her feelings for the Joker definitely originated during the sessions. But I like to think that she had more sessions with the Joker post-game that drove her to actually become Harley Quinn. The way I viewed it is that Origins just set the foundation and that the rest all happens in between Origins and Arkham Asylum.
Also, anyone who's interested in the scene to which we're referring in Arkham Origins, here's a link: