@Le_Flemard
Sure, he might be annoyed about the deception, but he experiences no loss from this trick. He doesn't care about the library, he's just doing community service and he's not happy about doing it and he stated it.
If you go back, you'll also notice that he's the one who mistakenly thought that the older woman was the witch and head librarian. Bessi just rolled with it, and believed his mistake was due to what Chris had told Justin about the library.
Honestly, if Justin is going to be pissed about someone, it's going to be about Chris. Even when he learns that Bessi is in fact the director, he may be surprised, and annoyed about the lie, but it's inherently something he just doesn't care about and that doesn't do him any harm. Whatever feeling it may bring, it won't be strong because this matter is unimportant. Besides, he may not know that Bessi is the Director, but she presented herself as the assistant of the Director, and since the start she's the one who has given him his tasks in the library. He's working under her either way.
The whole being angry to the point that he'll leave about being lied to? That's overblown. Even if he somehow really valued truth it would be overblown, but I guess it does kinda fit with the horrible personalities of the characters. (Let's be honest, the main characters are all dicks and it's just funny to look at the train wreck.)
Besides, he has little to say about tricking others and especially her. Dude's first encounter with her involved pretending to kiss her to remind her she's a weak woman and he's a strong man who can push her down. =_=