telling someone his SO is looking for another girl

Ba
I know its probably a "beating the dead horse topic" but if a guy you used to live with is engaged now and is keeps looking for you to have sex, should you tell his fiancé??? 
My friend is in that situation, his guy keeps looking for her and I tell her that she needs to tell his girl. Am I wrong for asking her to tell his girl? 
If I were that girl I would so definitely want to know! My sister said to not tell her, my husband and I agree on her to tell the girl. 
What is the right thing? 
I'm the one in green...to me it feels like she still wants him though :/ idk I just get these weird vibes!! 

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