Sometimes, we have to suffer or experience the consequences of our actions if we do something that is considered by many to be socially /morally unacceptable, the typical outcome is that many people will have a less than favorable opinion of us.
It is unfortunate that your daughter discovered what she did about her father's behavior. Having made the discovery it is unrealistic to not permit the child to form her own opinion. You should be insisting that the child should without fail but polite in the same way that she would be polite to any adult. There should be no expectation that she must like this person. It is likely in her young man, that she associates the pain and disruption experienced in her home with this person. Young or old people typically do not feel particularly positive towards people who have caused pain to their loved ones. That is pretty normal response.
The daughter is not attempting to dictate to whom the father can or cannot see. She is trying in her youthful way to indicate that she does not want to be in this person' s company.
The father is not being at all sensitive to his daughter's feelings. Given time, maturity, and appropriate circumstances, her feelings may change, but just as the daughter cannot control her father's romantic interests so too he cannot control or dictate his daughter's feelings.
Sometimes, the cost of getting what you want may include losing some of what you had. In the case , he has the partner that he wants but the cost may be damage to his relationship with his daughter. It need not be that way, but it does not appear that he is willing to invest the time and sensitivity that may be required to work through this situation.
OP, the best thing that you can do for your daughter is to insist that she be polite and remind her that regardless of what happened between her parents , you both love her very much even when it is difficult for her to believe that.