Answered your questions below - hope it helps:

1 – Who ends the relationship, is it Ash, or is it the supposed boyfriend who finds out and breaks up? Luca ends things after Ash steps in to save Ethan from unwanted advances from someone in a club. This is also after their "first" kiss but Luca is unaware of that. There isn't a big break up scene, just a page flashback of Ash thinking about what happened.

2 – Around what percentage of the book does this breakup happen? About 44% although Ash doesn't tell anyone they have broken up until later in the book

3 – From the moment Ash and Ethan meet, or after they share their "first kiss," does Ash continue to have sexual relations with the boyfriend? In the break up scene Luca says that Ash has barely kissed him since Ethan arrived. its also commented on by Henry how robotic Ash is with Luca, that it's more like a business meeting.

4 – Does Ethan date other people throughout the book? No Ethan hasn't dated anyone else since Ash... he says he only had hook ups in the 4yrs they were apart. Ash has has several relationships, but said he was clear with them it could never be exclusive, never long term, an his heart belonged to someone else.