You should have a lawyer represent your wife at the clerk's hearing. The quantum of proof for issuance of a criminal complaint is low, requiring only "probable cause". The facts you describe amount to assault and battery, which is defined as an intentional touching that was unconsented to.
The person who your wife pushed may have filed a private party application for criminal complaint, which mean that is he fails to appear for the clerk's hearing, the application will be denied. If the police applied for the criminal complaint, the alleged victim need not be present in order for the clerk to issue the complaint. In either situation, having a lawyer with you can help persuade the clerk that there is not probable cause to issue the complaint against your wife, and more importantly, can help you and your wife keep from making self-incriminating statements that could hurt your case should the matter proceed beyond the clerk's hearing.
As for whether your are able to find a witness, know that a witness is not essential to successfully defending the case. The burden of proof will be on the prosecutor to prove your wife guilty beyond all reasonable doubt, not on the defense to prove she is innocent. You already have two witnesses, you and your wife. Combined with good cross examination of the alleged victim and investigation into the alleged victim's past for prior convictions to be used as impeachment, this is a very defensible case.
Best of luck,
You should hire a lawyer to represent your wife at the Magistrate's hearing. The fact that this incident happened over nine months ago and the police were called at the time and spoke to the complainant tells me that this application for comaplint was likely filed by the "undesirable". I my experience the police would never wait that long to file an application for complaint.
What your wife received in the mail was probably just the summons to the hearing, which reveals very little information. The person filing the complaint, either police officer or "undesirable", would have had to fill out a statement of facts about the incident at the time it was filed. Your wife can obtain a copy of this statement of facts from the Clerk's office at the courthouse. Due process entitles her to know the facts that substantiate the allegation against her.
Clerk Magistrates are granted discretion at these hearings to "weed out" cases that do not merit issuing a complaint. To me this seems like just such a case. However, that being said the burden of proof is very low and anything your says at the hearing can be used against her later at a trial. Hiring a lawyer is a very good idea.
I agree with Attorney's Pang and Riddle, but I would also encourage you to go to the police station to obtain a copy of the report related to the incident. In case with a civilian filed complaint, not one filed by the police, the Clerk may not have a copy of it on the day of the hearing. If the police arrived and escorted the complainant off of your property without charging your wife, and the report reflects this, it will likely help to support your wife's case.
Disclaimer: This answer is provided for information purposes only and it is not intended as legal advice. Additionally, this answer does not create and attorney-client relationship. If you wish to obtain legal advice specific to your case, please consult with a local attorney.