Speaking from the stance of a broken home, where my mother cheated on my father, then divorced him and him disappearing from my life for a while. I can honestly say that you shouldn't take any tact that acknowledges his existence as her father.
If the father isn't willing to accept the child, as his, then he shouldn't be a factor in her life. I wouldn't mention it, until she is old enough to understand the responsibility of parenthood. I mean, when she is in school, she'll learn about parenting and even sooner from friends, and she will inquire/ask questions about her dad.
At that time, I would simply tell her the truth. She will love you more for being honest with her and it will allow her to deal with the adversity that comes with learning. It is important she learns about her father, but don't let her find out at a time where it can damage her future. You tell her too early, you'll do more damage than expected.