Non biological father signed Bc now wants custody after proving by DNA he is not the parent
She needs a lawyer. This is not a question for you to be asking random internet strangers. The answer would depend on a bazillion variables, and then it would be up to a judge.
I will say this:
Unless her ex (the one that is legally the father) has his rights terminated voluntarily, AND there is someone else to adopt, then yes, he can probably get joint custody. What kind of arrangement do they have now? Most judges will try to keep the status quo so that the child has stability, unless one parent is unfit. The ex willingly signed the birth certificate, so he is the father.
This comes across kind of harshly, but I was thinking along similar lines. I knew a woman whose boyfriend had raised her seven year old daughter since birth and the moment she left him she refused to let him see the girl (he was a GOOD father!) and when she'd say, "I want to see daddy," would tell her, "He is NOT your father!" That is just selfish and not okay and honestly quite harmful.
This happened to a man I know...he was the biological father and not told about the child's existence until the kid was 2. The non-biological father was given very minimal rights, and eventually just faded away. But he and the kid's mom divorced when the kid was under 3. My buddy had to fight to get to see his kid, and won, but he said it was a hard battle. The kid did not want to hang out with the non-biological dad, and it was confusing for him to have 2 dads. Be prepared for a legal battle...get a lawyer pronto. If the man has not been involved in the daughter's life, then I would question his motives, but if he has been involved in her life all along then he may be a stand-up guy and a good influence in her life.