Can anyone help me convince a friend that he cannot stop his fiance from seeing her daughter?

a friend of mine is engaged, and the marriage hangs on this issue: his fiance has a daughter, who in the past, has been a bit promiscuous, a drunk and a drug addict. he and his fiance' is a recovering alcoholics, and he is concerned of relaspes. he has forbidden his fiance from seeing her daughter, until she has proven she has changed. i have tried to convince him that this is a battle he can not win, but he refuses to "compromise". i am looking specifically for a poem or other writting that tells the story of a mother's unconditional love...i believe this just might be a lost cause, but i was asked to help, so now i am asking for help....