DNA

Hi i need some advice on DNA testing, My partner was with a girl 8 year ago, it was just a casual thing, after a short while she told him she was pregnant. When the little girl was born my partner took her on as his own, though he wasnt with the mother he had the baby every weekend and paid for her, and bonded with her. after a few weeks passed he was told of another man saying she was his child. So he confronted the mother and she didnt deny it nor confirm it, leaving my partner stumped, so he asked for a DNA test, to which she refused so he walked away. That wasthe hardest thing hes ever done.
Now weve been told that the little girl is asking about him and her mother has told her that he is her dad, also that she has been getting hit by her stepdad and now lives with her gran. Again we asked her for DNA and again she refused. I cant understand why she is refusing and telling the little girl that he is her dad.
We really need to know the truth because if it turnes out that he is her dad, he wants to see her and possible try for full custody.
What can we do if she wont permit a test. Is it possible to do a diy test without the mothers permission, really need help