Serena, noticing the heat-seeking missile of secrets pointed right at her, tries to evade D-Hump but to no avail. She should know by this time -- just ask Bart or Chuck, if you can't remember your own experiences -- that once Dan has decided it's his duty to poke and prod and pick at every possible part of your business you might as well just show him the books yourself. But because this secret is a double-secret with a secret twist -- being less a secret than a dual-layered cover story for a much larger secret that threatens, think about it, not only her mother's livelihood but the Rufus/Lily relationship and thus Dan and Jenny's ultimate welfare, not to mention how much a bitch Chuck will be about it -- the math is simply too complex for Serena, and she's snatched up in a split-second like a hare in a trap: "Serena, there was no ashram!"
She admits that there was no ashram, but asks him to stay out of the lies she tells her family, and Dan makes a valid point that A) he is her family and B) she is also lying to him as Dan qua Dan, having written him many letters of colonics and bikram yoga that turned out to have been lifted straight from Gwyneth's newsletter Goop, which is the best joke in the entire episode, because now that Drew Fucking Barrymore's doing a fucking roller derby movie with Ellen Fucking Page and they're kissing in magazines, Gwyneth could actually escape her decade-long onus of being The Worst by simply acting more like Serena and less like Vanessa, but she doesn't have it in her and if she did her weakened arms couldn't pull hard enough to get it out.
So Dan's like, "What's really going on?" and because again, this episode is built on people having conversations that don't make sense so that the end of the episode will blow your mind, she doesn't have a meaningful answer to this meaningless question, and he shows her pictures from "all over the internet" which he happens to have on his phone that depict her doing blowsy slutty things like laughing and drinking beverages fully clothed, and she gets the fuck up out of there as well she should, because without Lily there the entire world is going to revolve around Dan's misplaced and creepy sense of justice, and you have a limited window. Dan is, of course, deeply disappointed in Serena. Both as a sister and a friend, but mostly as a human being.