Man cheating while their partner’s pregnant

Why do women stay with a guy that cheated on them while pregnant or after giving birth?

My friend's boyfriend cheated on her for weeks with his ex while she was taking care of her newborn baby. When she found out he was cheating her baby was two weeks ago. After he apologized few times she took him back. Now I have a difficult time taking her advice regarding relationships. I know it isn’t her problem it's mine.