Dave's mercurial irony evinces a genuine anxiety or distress about his own artistic morality, which battens on disaster and misery and fortuitously makes money from the wreckage of other people's lives. What he sees in David, I think, is a replica of himself - it takes one to know one. Of course, David is a less talented and successful facsimile. In fact, both Dave and David were case-hardened harlots. This was no feckless fling, but a shrewd commercial transaction.
Celebrities only give interviews when they have something to sell.
This was the hooker's unfelt rhetoric of solicitation and encouragement. Dave's chastening of David is not about ethics at all, since morally they're on a par. It's about the supposedly superior rights of the celebrity. Dave's fame exempts him from the customary moral laws, and from emotional qualms that we might otherwise feel about his behaviour. His book turns his family tragedy into a soap opera.