I’m not an expert on loss for a child so young. I can imagine that she will experience her LF’s absence for a time since he has been so involved with her.
I’m guessing that he is leaving due to conflict in your relationship with him. The effect of the loss of her “dad” may be outweighed by the loss of the stress in the house. A positive and nurturing relationship with you is the most valuable protective factor while you both go through this difficult transition.
That being said, it is not the baby’s fault that the two of you can’t be a couple. Since he’s been a good dad to her, why not explore with him the possibility that he would continue to have a role in her life. He may have talked about rejecting her in anger with you; not because he really means it.
A caring father figure is good for any child — as long as it is a real long-term commitment. If the two of you can separate any negative feelings about each other from your love for the baby, you might all benefit.
I wish you well.