When our partner turn down in a relationships I think there were many aspects. Did your friend treated her partner right? did your friend focus to upgrade herself? did your friend think she can get anything she wants just because she already had sex?

Values of a relationships is not just about sex. There's other connection to build in it. If she think value of her relationship is just about sex, she might be wrong and not ready to be in relationship. It's a reason why she left in bad terms.