Happens in the game, need to max out "holly_change" (keep her going to the gym, encourage her to be more outgoing), agree with Ivy that she should start texting with Mark, and don't have Ian and Holly dating. I'm not sure off memory if you also need to keep Lena and Holly a bit apart (they can have sex with each other for sure, just don't remember if they getting too romantic would block the scene).
If everything is setup right, Lena and Ivy can encourage her to go be with Mark during Ivy's birthday, and if her "holly_change" variable was really high then Lena should receive that picture from Ivy later.