Uh, what? Which heroine? I don't remember such thing happens so far? Though there was that one girl who fell in love with his retainer(?), I'm pretty sure she wasn't a heroine.
Did you want to say netorare? Spoiler: NTR NTR Japanese acronym for Netorare, used to define a genre of Eroge (Hentai Game), meaning "Cuckold". In short, the main protagonist's loved one(s) are taken or seduced away from him and the heroine might be willing or unwilling. This is to cause an emotion of deep jealousy on the reader.