I am friends with a single Muslim male. We are thinking of future marriage.
He has an adopted daughter. She was 10 years old when he adopted her and she is now 18 years old.
She lives with her mother and younger brother in a house that he brought for them. He pays for all costs for the house including power, food, renovations and all expenses for daughter.
He visits the home alone and takes daughter out and sometimes mother and brother as well altogether on picnic. Maybe overnight picnic soon.
He feeds daughter food, and she oils his beard and sleeps on his chest.
I expressed that I do not think all of this is ok, especially as she is 18 years old and he is caring for whole family, it doesn't seem right to me.
He said this is the way it is in his culture and it will not change. She is his daughter and will always come first. She is like a child, not a women. She has some issues.
He will.continue to visit and take her out and that him and daughter are very close and I should not try to change anything about their relationship.
Daughter also has a jinn now, so he needs to spend more time with her to help healing.
If daughter is not ok to speak on phone because of her sickness when is is away, he talks to daughters mother, most times twice per day.
He said there is no intimate relationship with daughters mother, it is formal.
Is this an acceptable relationship with daughter and daughters mother in Islam?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
At the outset, we wish to point out that your friendship with this man in reference is not permissible. Although you state that you are considering marriage with him in the future, he is currently a stranger (non-Mahram) to you. It is absolutely forbidden to unnecessarily interact with a non-Mahram man.
With regards to his adopted daughter and her mother, both must do Hijab and Purdah in his presence, as both are non-Mahram to him (women with whom marriage is permissible).  Furthermore, he is not allowed to unnecessarily interact with the adopted daughter nor her mother, as unnecessary interaction with non-Mahram females is impermissible.
And Allah Ta’āla Knows Best
Mufti Abdul Azeem bin Abdul Rahman Khan
 Fatawa Uthmani, Vol. 2, page 484, Maktabah Ma’ariful Quran Karachi