This is a very subjective question. Ultimately, the minimum evidence required for a conviction is whatever the assigned judge at trial (assuming this is in juvenile court) subjectively believes is needed to convince him/her beyond a reasonable doubt that the juvenile committed the crime.
In the instant case, it sounds as though the juvenile may have some good arguments to present in pretrial negotiations and at trial. If the firearms were never recovered, that could potentially create additional issues for the prosecution. If the judge finds the accomplice credible at trial, and believes the the juvenile did in fact take the firearms during the burglary, there would still need to be evidence that the guns taken do in fact qualify as "firearms" as statutorily defined (typically proven by having law enforcement test fire the weapon, but the State could potentially prove this in another manner depending on the specific facts present in your case).
There will always be risk involved in going to trial, and it sounds as though this trial may turn in large part on credibility issues. These are serious charges, so the juvenile should make sure he has an experienced defense attorney to assist him through the process.
If you have any additional questions, please free to get in touch with my law office via email or phone.
Jeff Holmes - Attorney at Law - email@example.com - 360.975.9288. Disclaimer: This answer does not constitute legal advice. This information is based on general principles of law, as well as my general experience that may or may not relate to your specific situation. This information is not meant to take the place of actually consulting an Attorney in your jurisdiction. If you would like legal advice, I would recommend consulting an attorney in your locale.
The friend's report cannot be used to establish guilt cannot be used at court to establish guilt unless he is willing to testify and subject himself to incrimination. That said they do not have to find the guns to make an arrest. In this case it sounds to me like they have probable cause to make an arrest on the burglary and the firearms theft w/o the friend's report and without the guns.