Let´s see if I can explain it properly (If I have problems expresing in spanish what is complex on my mind, in english is a pain in the ass). The first modelling happens in chapter one, which is sort of a prologue for setting the characters, their contexts and personalities. That happens cause that´s what Lena does and the way Eva Kiss wants to strength the link between Lena, Ian and other characters, besides the café. I would understand better the critique if the second nude modelling event was unavoidable but the first one belongs to the own story context. It´s like asking if Ian can be a bank worker instead of a writer/worker of a book agency.