The BEST advice I can give is to get a criminal defense attorney. You don't want to waive any defenses or not respond appropriately in an armed robbery case where he is facing jail time.
One thing I would suggest is that in an armed robbery, a necessary element the prosecutor has to prove, beyond all reasonable doubt, is that there was a weapon involved. If there is no evidence at all of a weapon, then maybe move to dismiss the case based on lack of evidence on a necessary element of the crime. However, if any of the co-defendants even mention a weapon, that there is some small piece of evidence and that might be enough.
But based on the details you listed above, please don't try to handle this on your own. Armed Robbery is a serious offense and you should definitely consult with an experienced criminal defense attorney.
Sorry, but the only advice to give is to "hire a lawyer." Trying to explain to you how to defend this case or "the best defense" is simply not possible with the limited facts you provided, without talking to your friend and without reviewing the discovery. Additionally, there are hundreds of variables that can change on any given moment when you are in court defending someone charged with a crime. Most of the lawyers who take the time to answer questions like yours have been practicing law for a long time; we would not be able to competently explain how to represent someone charged with such a serious offense to a lawyer who just passed the bar, let alone a non-lawyer such as yourself. That is because our job requires lots and lots of experience and you can only get that experience by doing our job. I currently represent a person who was in a similar situation as your friend. Instead of hiring an experienced lawyer, he hired a cheap one who said "there is no way you will get convicted." The problem is the cheap lawyer did not conduct any investigation and did not know what he was doing at trial. Now my client is serving 10 years in prison and we are in the process of getting him a new trial. If you can't afford a lawyer, apply for the public defender. Your friends lawyer will figure out the best strategy for defending the case.
The advice is to hire a lawyer. You can't practice law so you can't advise him. Only a lawyer can advise him as to what to do.
An armed robbery conviction has a minimum penalty of 10 years in prison (no parole). Your friend is facing a very serious charge. Although you stated you want some advise other than hiring a lawyer, that is what your friend needs - an experienced criminal defense lawyer.