I would argue the signs were there a lot earlier than that, the way he behaves to his odd first friend at Uni after getting in with Felix is brutal, and even at the time, whilst I won't pretend I definitely knew he was lying, the father dying right after it appears Felix is trying to drop him seemed a bit of a convenient sob story. Also, if you were really, really paying attention (I found out after), when the coronation anthem Zadok the Priest is sung at the start the choir are saying Oliver Quick, a pretty major clue that the character is not what he appears to be. Oh and the fact he's middle class absolutely does not undercut the class commentary, there's no one more desperate than the middle class trying to get into the upper class.
But as I said in a previous answer, this film definitely isn't trying to be a Highsmith novel, the comedy is way too central for that. More generally there's an interesting gender divide with Fennell's work, with men a lot more critical of it than women for some reason.
