My name is ***** ***** I am an experienced criminal lawyer.
Defense attorneys and prosecutors work together daily and frequently establish cordial professional relationships. That's in most cases good for a defense, as I'm sure you've heard the old saying that you can catch more flies with honey than with vinegar. If you want a prosecutor to do you favors, it pays to treat them well. Seeing a DA and a defense lawyer chatting companionably while waiting for a case to be called is very normal and doesn't mean there's a conflict of interest.
However, if you know that the DA and the defense lawyer are very close friends, then that may be a conflict of interest. In that case, your father can contact the public defender organization, talk to his public defender's supervisor, explain the conflict and ask if a different public defender can be substituted. If the supervisor says no, or if the public defender is in private practice and has no supervisor, then on the next court date your father can talk to the judge about the conflict and ask that he be relieved from the case and a new public defender appointed.
As for the rest, the wheels of the criminal justice system grind very slowly. If the defendant is not interested in a plea bargain and wants to fight this case, the defendant's case could be pending for a year or more before it actually gets to trial. But your father is the complainant, meaning he is the single most important witness on this case and the prosecutor is representing his interest. The prosecutor is not going to need him in court until trial, if the case gets that far. But he has a right to know what's going on with the prosecution, and if he is being harassed by the defense lawyer, he can also complain to the prosecutor about that.
He needs to call the prosecutor's office, ask to speak to the prosecutor in charge of the case against the defendant and give him a call. He should tell the prosecutor he's the complainant in the case and wants to know what's going on. Your father has no obligation incidentally to talk to the defense lawyer. He can refuse to speak to him and should, in the future, simply walk away and call the prosecutor and report this.