The short answer to your questions is the evidence has to be sufficient to prove to a jury beyond a reasonable doubt that your friend committed the crime. That doesn't give you much to work with as a practical matter and it's just not possible for us to tell you exactly what to do to disprove the accusation as we don't know all the facts, so the longer answer is that you need to either hire a lawyer or make sure your friend quickly applies for a public defender so that they can get some help with the case. You are right that a rape charge is very serious and it could cause the parole to be revoked even if they aren't convicted of the crime.
This answer is provided for informational purposes only. The response given is not intended to create, nor does it create, an attorney-client relationship. Each case is fact specific, and it is impossible to evaluate a legal question without a review of all the facts at issue. If you are looking for legal advice regarding a problem in the State of Georgia you may contact my office so that we can set up an office or phone consultation to fully address your question. http://www.kimmeylaw.com
If your friend cannot afford to hire an attorney, he should immediately applly for a court appointed attorney. The charge your friend faces is one that often turns on the credibiltiy of the witnesses and he needs someone investigating the facts of the case as soon as possible and that person needs to be a lawyer.