What to DO?

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Tell your friend to read up on domestic violence, then seek help from a woman's refuge.
If it for the sake of being so called 'married' which is expected of a Nigerian woman at a certain age i think she does need to re assess her marital responsibility and leave.
The bottom line is it is up to her to get help . Her partner to seek the same and to address personal issues he has causing him to be weak in vital areas of his life , but finds strength when he dominates and beats a woman.
Domestic violence can ultimately lead to Death or murder but it generally called manslaughter, however the person who has been battered is dead and the family is left to mourn.
Encourage her to read this and point her in the right direction.
God bless