Why does my ex boyfriend ex wants to be friends with me?

Ok I was with my ex in october last year we broke up because of his narcissistic ways..didnt talk until april this year..had didnt know he was in a relationship until he was saying "i love you" to this girl he was dating and was seeing around the same time last year as me..i told her this year april when they was together he had with me ..and hes a jerk who will leave you..she said thankyou but im staying..we started talking again this month her and I and she said he and her are nolonger together..he flirts and disrespects her by walking off..then she started to flirt with me..i found out her age and shes only 17..hes 21..i was shocked im 28..i told her I cant beleive he was with a 16 yearold when we was talking in october..well her and him are back friends and I blocked her..she begged me to still be her friend..why does this girl want to be my friend so bad and start back being friends with him?